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COBOL Tutorial

COBOL CENTURY-DAY Function

The CENTURY-DAY intrinsic function in COBOL converts Julian dates by adding century information to two-digit year formats, enabling proper handling of Julian date calculations across century boundaries. This function is essential for astronomical calculations, scientific applications, date arithmetic operations, and legacy system modernization where Julian date formats are used for precise day-of-year calculations and chronological sequencing in applications requiring high-precision date manipulation and century-aware Julian date processing.

Understanding CENTURY-DAY Function

CENTURY-DAY converts a Julian date from YYDDD format to YYYYDDD format using specified century windowing rules. Julian dates represent the day of the year (1-366) and are commonly used in scientific, astronomical, and some business applications for precise date calculations.

Key Features:

  • Julian date century conversion
  • Day-of-year precision handling
  • Scientific date format support
  • Century windowing for Y2K compliance
  • Astronomical calculation compatibility
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DATA DIVISION.
WORKING-STORAGE SECTION.
01  JULIAN-DATE-PROCESSING.
    05  INPUT-JULIAN-YY    PIC 9(5) VALUE 99365.  *> YYDDD (Dec 31, 1999)
    05  OUTPUT-JULIAN-YYYY PIC 9(7).              *> YYYYDDD
    05  PIVOT-YEAR         PIC 99 VALUE 30.
    05  DAY-OF-YEAR        PIC 9(3).
    05  CENTURY            PIC 99.
 
01  JULIAN-EXAMPLES.
    05  NEW-YEAR-2000      PIC 9(5) VALUE 00001.  *> Jan 1, 2000
    05  LEAP-DAY-2000      PIC 9(5) VALUE 00060.  *> Feb 29, 2000
    05  MILLENNIUM-END     PIC 9(5) VALUE 99365.  *> Dec 31, 1999
    05  LEAP-YEAR-1996     PIC 9(5) VALUE 96366.  *> Dec 31, 1996
 
PROCEDURE DIVISION.
BASIC-CENTURY-DAY-CONVERSION.
    *> Basic century day conversion
    COMPUTE OUTPUT-JULIAN-YYYY = FUNCTION CENTURY-DAY(INPUT-JULIAN-YY)
    DISPLAY "Original Julian (YYDDD): " INPUT-JULIAN-YY
    DISPLAY "Century Julian (YYYYDDD): " OUTPUT-JULIAN-YYYY
    
    *> Extract day of year
    COMPUTE DAY-OF-YEAR = FUNCTION MOD(OUTPUT-JULIAN-YYYY, 1000)
    COMPUTE CENTURY = OUTPUT-JULIAN-YYYY / 1000
    DISPLAY "Century: " CENTURY " Day of year: " DAY-OF-YEAR
    
    *> Process various Julian date examples
    PERFORM CONVERT-JULIAN-EXAMPLES.
 
CONVERT-JULIAN-EXAMPLES.
    COMPUTE OUTPUT-JULIAN-YYYY = FUNCTION CENTURY-DAY(NEW-YEAR-2000)
    DISPLAY "New Year 2000: " OUTPUT-JULIAN-YYYY
    
    COMPUTE OUTPUT-JULIAN-YYYY = FUNCTION CENTURY-DAY(LEAP-DAY-2000)
    DISPLAY "Leap Day 2000: " OUTPUT-JULIAN-YYYY
    
    COMPUTE OUTPUT-JULIAN-YYYY = FUNCTION CENTURY-DAY(MILLENNIUM-END)
    DISPLAY "Millennium End: " OUTPUT-JULIAN-YYYY.

Scientific and Astronomical Applications

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*> Scientific applications using CENTURY-DAY
DATA DIVISION.
WORKING-STORAGE SECTION.
01  ASTRONOMICAL-CALCULATIONS.
    05  OBSERVATION-DATE   PIC 9(5).
    05  CENTURY-OBS-DATE   PIC 9(7).
    05  JULIAN-DAY-NUMBER  PIC 9(8).
    05  MODIFIED-JULIAN    PIC 9(7).
    05  DAYS-SINCE-EPOCH   PIC 9(6).
 
01  SATELLITE-TRACKING.
    05  LAUNCH-DATE        PIC 9(5) VALUE 96001.  *> Jan 1, 1996
    05  ORBIT-PERIOD       PIC 9(3)V99 VALUE 090.5.
    05  CURRENT-POSITION   PIC 9(5)V99.
    05  MISSION-DURATION   PIC 9(4).
 
01  SCIENTIFIC-CONSTANTS.
    05  JULIAN-EPOCH       PIC 9(8) VALUE 17210101.  *> Jan 1, 1721
    05  MODIFIED-JD-EPOCH  PIC 9(8) VALUE 18581117.  *> Nov 17, 1858
    05  UNIX-EPOCH         PIC 9(8) VALUE 19700101.  *> Jan 1, 1970
 
PROCEDURE DIVISION.
SCIENTIFIC-DATE-PROCESSING.
    PERFORM ASTRONOMICAL-DATE-CONVERSION
    PERFORM SATELLITE-ORBIT-CALCULATIONS
    PERFORM MODIFIED-JULIAN-DAY-CALC.
 
ASTRONOMICAL-DATE-CONVERSION.
    *> Convert observation dates for astronomical calculations
    MOVE 23045 TO OBSERVATION-DATE  *> Day 45 of year 23 (2023)
    COMPUTE CENTURY-OBS-DATE = 
        FUNCTION CENTURY-DAY(OBSERVATION-DATE, 30)
    
    DISPLAY "Observation Date Processing:"
    DISPLAY "Input Julian: " OBSERVATION-DATE
    DISPLAY "Century Julian: " CENTURY-OBS-DATE
    
    *> Calculate Julian Day Number (simplified)
    PERFORM CALCULATE-JULIAN-DAY-NUMBER
    DISPLAY "Julian Day Number: " JULIAN-DAY-NUMBER.
 
CALCULATE-JULIAN-DAY-NUMBER.
    *> Simplified Julian Day calculation
    COMPUTE WS-YEAR = CENTURY-OBS-DATE / 1000
    COMPUTE WS-DAY = FUNCTION MOD(CENTURY-OBS-DATE, 1000)
    
    *> Basic JDN calculation (simplified formula)
    COMPUTE JULIAN-DAY-NUMBER = 
        (1461 * (WS-YEAR + 4800)) / 4 +
        WS-DAY - 32045.
 
SATELLITE-ORBIT-CALCULATIONS.
    *> Calculate satellite position based on launch date
    COMPUTE CENTURY-OBS-DATE = FUNCTION CENTURY-DAY(LAUNCH-DATE, 50)
    
    DISPLAY "Satellite Tracking:"
    DISPLAY "Launch Date: " CENTURY-OBS-DATE
    
    *> Calculate mission duration in days
    COMPUTE WS-CURRENT-JULIAN = FUNCTION CENTURY-DAY(23100, 30)  *> Day 100 of 2023
    COMPUTE MISSION-DURATION = 
        (WS-CURRENT-JULIAN - CENTURY-OBS-DATE) / 1000
    
    DISPLAY "Mission Duration: " MISSION-DURATION " days"
    
    *> Calculate current orbital position
    COMPUTE CURRENT-POSITION = 
        FUNCTION MOD(MISSION-DURATION / ORBIT-PERIOD, 1) * 360
    DISPLAY "Current Orbital Position: " CURRENT-POSITION " degrees".
 
MODIFIED-JULIAN-DAY-CALC.
    *> Calculate Modified Julian Day
    COMPUTE MODIFIED-JULIAN = JULIAN-DAY-NUMBER - 2400000
    DISPLAY "Modified Julian Day: " MODIFIED-JULIAN.

Date Arithmetic and Calculations

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*> Advanced date arithmetic using CENTURY-DAY
DATA DIVISION.
WORKING-STORAGE SECTION.
01  DATE-ARITHMETIC-WORKSPACE.
    05  START-JULIAN       PIC 9(5) VALUE 23001.  *> Jan 1, 2023
    05  END-JULIAN         PIC 9(5) VALUE 23365.  *> Dec 31, 2023
    05  CENTURY-START      PIC 9(7).
    05  CENTURY-END        PIC 9(7).
    05  DURATION-DAYS      PIC 9(4).
    05  BUSINESS-DAYS      PIC 9(4).
    05  WEEKEND-DAYS       PIC 9(3).
 
01  QUARTER-CALCULATIONS.
    05  Q1-START           PIC 9(7) VALUE 2023001.  *> Jan 1
    05  Q1-END             PIC 9(7) VALUE 2023090.  *> Mar 31
    05  Q2-START           PIC 9(7) VALUE 2023091.  *> Apr 1
    05  Q2-END             PIC 9(7) VALUE 2023181.  *> Jun 30
    05  Q3-START           PIC 9(7) VALUE 2023182.  *> Jul 1
    05  Q3-END             PIC 9(7) VALUE 2023273.  *> Sep 30
    05  Q4-START           PIC 9(7) VALUE 2023274.  *> Oct 1
    05  Q4-END             PIC 9(7) VALUE 2023365.  *> Dec 31
 
01  FISCAL-YEAR-TRACKING.
    05  FISCAL-START       PIC 9(7).
    05  FISCAL-END         PIC 9(7).
    05  FISCAL-QUARTERS    OCCURS 4 TIMES.
        10  FQ-START       PIC 9(7).
        10  FQ-END         PIC 9(7).
        10  FQ-DAYS        PIC 9(3).
 
01  LEAP-YEAR-ANALYSIS.
    05  LEAP-INDICATORS    OCCURS 20 TIMES.
        10  LY-YEAR        PIC 9(4).
        10  LY-IS-LEAP     PIC X VALUE 'N'.
            88  IS-LEAP-YEAR VALUE 'Y'.
        10  LY-MAX-DAY     PIC 9(3).
 
PROCEDURE DIVISION.
ADVANCED-DATE-ARITHMETIC.
    PERFORM CALCULATE-DATE-RANGES
    PERFORM FISCAL-YEAR-CALCULATIONS
    PERFORM LEAP-YEAR-ANALYSIS
    PERFORM BUSINESS-DAY-CALCULATIONS.
 
CALCULATE-DATE-RANGES.
    *> Convert Julian dates to century format
    COMPUTE CENTURY-START = FUNCTION CENTURY-DAY(START-JULIAN, 30)
    COMPUTE CENTURY-END = FUNCTION CENTURY-DAY(END-JULIAN, 30)
    
    DISPLAY "Date Range Analysis:"
    DISPLAY "Start: " CENTURY-START
    DISPLAY "End: " CENTURY-END
    
    *> Calculate duration
    COMPUTE WS-START-YEAR = CENTURY-START / 1000
    COMPUTE WS-START-DAY = FUNCTION MOD(CENTURY-START, 1000)
    COMPUTE WS-END-YEAR = CENTURY-END / 1000
    COMPUTE WS-END-DAY = FUNCTION MOD(CENTURY-END, 1000)
    
    IF WS-START-YEAR = WS-END-YEAR
        COMPUTE DURATION-DAYS = WS-END-DAY - WS-START-DAY + 1
    ELSE
        COMPUTE DURATION-DAYS = 
            (365 - WS-START-DAY) + WS-END-DAY + 1 +
            ((WS-END-YEAR - WS-START-YEAR - 1) * 365)
    END-IF
    
    DISPLAY "Duration: " DURATION-DAYS " days".
 
FISCAL-YEAR-CALCULATIONS.
    *> Define fiscal year boundaries (Oct 1 - Sep 30)
    MOVE 2022274 TO FISCAL-START    *> Oct 1, 2022
    MOVE 2023273 TO FISCAL-END      *> Sep 30, 2023
    
    *> Calculate fiscal quarters
    MOVE FISCAL-START TO FQ-START(1)
    COMPUTE FQ-END(1) = FISCAL-START + 91   *> ~Q1
    COMPUTE FQ-START(2) = FQ-END(1) + 1     *> Q2 start
    COMPUTE FQ-END(2) = FQ-START(2) + 90    *> ~Q2
    COMPUTE FQ-START(3) = FQ-END(2) + 1     *> Q3 start
    COMPUTE FQ-END(3) = FQ-START(3) + 91    *> ~Q3
    COMPUTE FQ-START(4) = FQ-END(3) + 1     *> Q4 start
    MOVE FISCAL-END TO FQ-END(4)            *> Q4 end
    
    DISPLAY "Fiscal Year Quarters:"
    PERFORM VARYING WS-Q FROM 1 BY 1 UNTIL WS-Q > 4
        COMPUTE FQ-DAYS(WS-Q) = 
            FUNCTION MOD(FQ-END(WS-Q), 1000) - 
            FUNCTION MOD(FQ-START(WS-Q), 1000) + 1
        DISPLAY "Q" WS-Q ": " FQ-START(WS-Q) " - " FQ-END(WS-Q) 
                " (" FQ-DAYS(WS-Q) " days)"
    END-PERFORM.
 
LEAP-YEAR-ANALYSIS.
    *> Analyze leap years and maximum days
    PERFORM VARYING WS-YEAR FROM 2020 BY 1 UNTIL WS-YEAR > 2039
        COMPUTE WS-IDX = WS-YEAR - 2019
        MOVE WS-YEAR TO LY-YEAR(WS-IDX)
        
        *> Check if leap year
        IF FUNCTION MOD(WS-YEAR, 4) = 0
            IF FUNCTION MOD(WS-YEAR, 100) = 0
                IF FUNCTION MOD(WS-YEAR, 400) = 0
                    MOVE 'Y' TO LY-IS-LEAP(WS-IDX)
                    MOVE 366 TO LY-MAX-DAY(WS-IDX)
                ELSE
                    MOVE 'N' TO LY-IS-LEAP(WS-IDX)
                    MOVE 365 TO LY-MAX-DAY(WS-IDX)
                END-IF
            ELSE
                MOVE 'Y' TO LY-IS-LEAP(WS-IDX)
                MOVE 366 TO LY-MAX-DAY(WS-IDX)
            END-IF
        ELSE
            MOVE 'N' TO LY-IS-LEAP(WS-IDX)
            MOVE 365 TO LY-MAX-DAY(WS-IDX)
        END-IF
        
        DISPLAY LY-YEAR(WS-IDX) ": " LY-IS-LEAP(WS-IDX) 
                " (" LY-MAX-DAY(WS-IDX) " days)"
    END-PERFORM.
 
BUSINESS-DAY-CALCULATIONS.
    *> Calculate business days excluding weekends
    MOVE ZERO TO BUSINESS-DAYS WEEKEND-DAYS
    
    PERFORM VARYING WS-JULIAN-DAY FROM CENTURY-START BY 1
        UNTIL WS-JULIAN-DAY > CENTURY-END
        
        *> Convert Julian to regular date for day-of-week calc
        PERFORM CONVERT-JULIAN-TO-GREGORIAN
        PERFORM CALCULATE-DAY-OF-WEEK
        
        IF WS-DAY-OF-WEEK = 1 OR WS-DAY-OF-WEEK = 7
            ADD 1 TO WEEKEND-DAYS
        ELSE
            ADD 1 TO BUSINESS-DAYS
        END-IF
    END-PERFORM
    
    DISPLAY "Business Days: " BUSINESS-DAYS
    DISPLAY "Weekend Days: " WEEKEND-DAYS.

Century Windowing and Y2K Compliance

Century windowing is crucial for proper CENTURY-DAY function behavior. The function uses a pivot year to determine which century a two-digit year belongs to. Years equal to or greater than the pivot belong to the earlier century, while years less than the pivot belong to the later century.

Windowing Strategies:

  • Fixed window: Use a consistent pivot year (e.g., 30 for 1930-2029)
  • Sliding window: Adjust pivot based on current year
  • Business rule window: Use domain-specific logic for century assignment
  • Explicit century: Avoid ambiguity by using four-digit years when possible
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*> Century windowing strategies for CENTURY-DAY
DATA DIVISION.
WORKING-STORAGE SECTION.
01  WINDOWING-EXAMPLES.
    05  PIVOT-YEAR-30      PIC 99 VALUE 30.
    05  PIVOT-YEAR-50      PIC 99 VALUE 50.
    05  PIVOT-YEAR-80      PIC 99 VALUE 80.
    05  SLIDING-PIVOT      PIC 99.
 
01  TEST-JULIAN-DATES.
    05  TEST-DATES         OCCURS 10 TIMES.
        10  TJD-INPUT      PIC 9(5).
        10  TJD-PIVOT30    PIC 9(7).
        10  TJD-PIVOT50    PIC 9(7).
        10  TJD-PIVOT80    PIC 9(7).
        10  TJD-SLIDING    PIC 9(7).
 
01  BUSINESS-WINDOWING.
    05  BIRTH-DATE-PIVOT   PIC 99 VALUE 25.  *> People born 1925-2024
    05  CONTRACT-PIVOT     PIC 99 VALUE 70.  *> Contracts 1970-2069
    05  TRANSACTION-PIVOT  PIC 99 VALUE 90.  *> Transactions 1990-2089
 
PROCEDURE DIVISION.
CENTURY-WINDOWING-DEMO.
    PERFORM INITIALIZE-TEST-DATES
    PERFORM COMPARE-WINDOWING-STRATEGIES
    PERFORM BUSINESS-RULE-WINDOWING
    PERFORM SLIDING-WINDOW-DEMO.
 
INITIALIZE-TEST-DATES.
    MOVE 99365 TO TJD-INPUT(1)  *> Dec 31, 1999 or 2099?
    MOVE 00001 TO TJD-INPUT(2)  *> Jan 1, 1900 or 2000?
    MOVE 25180 TO TJD-INPUT(3)  *> Day 180, 1925 or 2025?
    MOVE 50366 TO TJD-INPUT(4)  *> Dec 31, 1950 or 2050? (leap year)
    MOVE 80001 TO TJD-INPUT(5)  *> Jan 1, 1980 or 2080?
    MOVE 19100 TO TJD-INPUT(6)  *> Day 100, 1919 or 2019?
    MOVE 75200 TO TJD-INPUT(7)  *> Day 200, 1975 or 2075?
    MOVE 30060 TO TJD-INPUT(8)  *> Feb 29, 1930 or 2030?
    MOVE 95001 TO TJD-INPUT(9)  *> Jan 1, 1995 or 2095?
    MOVE 10365 TO TJD-INPUT(10) *> Dec 31, 1910 or 2010?.
 
COMPARE-WINDOWING-STRATEGIES.
    DISPLAY "Century Windowing Comparison:"
    DISPLAY "Input   Pivot30  Pivot50  Pivot80"
    DISPLAY "======================================="
    
    PERFORM VARYING WS-I FROM 1 BY 1 UNTIL WS-I > 10
        *> Test with different pivot years
        COMPUTE TJD-PIVOT30(WS-I) = 
            FUNCTION CENTURY-DAY(TJD-INPUT(WS-I), PIVOT-YEAR-30)
        COMPUTE TJD-PIVOT50(WS-I) = 
            FUNCTION CENTURY-DAY(TJD-INPUT(WS-I), PIVOT-YEAR-50)
        COMPUTE TJD-PIVOT80(WS-I) = 
            FUNCTION CENTURY-DAY(TJD-INPUT(WS-I), PIVOT-YEAR-80)
            
        DISPLAY TJD-INPUT(WS-I) "  " 
                TJD-PIVOT30(WS-I) " " 
                TJD-PIVOT50(WS-I) " " 
                TJD-PIVOT80(WS-I)
    END-PERFORM.
 
BUSINESS-RULE-WINDOWING.
    DISPLAY "Business Rule Windowing Examples:"
    
    *> Birth date processing
    MOVE 75180 TO WS-BIRTH-JULIAN  *> Day 180, year 75
    COMPUTE WS-BIRTH-CENTURY = 
        FUNCTION CENTURY-DAY(WS-BIRTH-JULIAN, BIRTH-DATE-PIVOT)
    DISPLAY "Birth Date (75180): " WS-BIRTH-CENTURY
    
    *> Contract date processing
    MOVE 85090 TO WS-CONTRACT-JULIAN  *> Day 90, year 85
    COMPUTE WS-CONTRACT-CENTURY = 
        FUNCTION CENTURY-DAY(WS-CONTRACT-JULIAN, CONTRACT-PIVOT)
    DISPLAY "Contract Date (85090): " WS-CONTRACT-CENTURY
    
    *> Transaction date processing
    MOVE 95001 TO WS-TRANSACTION-JULIAN  *> Day 1, year 95
    COMPUTE WS-TRANSACTION-CENTURY = 
        FUNCTION CENTURY-DAY(WS-TRANSACTION-JULIAN, TRANSACTION-PIVOT)
    DISPLAY "Transaction Date (95001): " WS-TRANSACTION-CENTURY.
 
SLIDING-WINDOW-DEMO.
    *> Calculate sliding pivot based on current year
    MOVE FUNCTION CURRENT-DATE(1:4) TO WS-CURRENT-YEAR
    COMPUTE SLIDING-PIVOT = FUNCTION MOD(WS-CURRENT-YEAR + 30, 100)
    
    DISPLAY "Sliding Window Demo:"
    DISPLAY "Current Year: " WS-CURRENT-YEAR
    DISPLAY "Sliding Pivot: " SLIDING-PIVOT
    
    *> Apply sliding window to test dates
    PERFORM VARYING WS-I FROM 1 BY 1 UNTIL WS-I > 5
        COMPUTE TJD-SLIDING(WS-I) = 
            FUNCTION CENTURY-DAY(TJD-INPUT(WS-I), SLIDING-PIVOT)
        DISPLAY "Input: " TJD-INPUT(WS-I) 
                " Sliding: " TJD-SLIDING(WS-I)
    END-PERFORM.

Performance Optimization and Best Practices

CENTURY-DAY Optimization Guidelines:

  • Use consistent pivot years throughout your application for predictable behavior
  • Cache CENTURY-DAY results when processing multiple dates with the same pivot
  • Validate Julian date ranges (001-365/366) before conversion to prevent errors
  • Consider using four-digit years in new applications to avoid century ambiguity
  • Document your century windowing strategy for maintenance and compliance
  • Test century boundary conditions thoroughly, especially around year 2000
  • Use business rules to determine appropriate pivot years for different data types
  • Implement error handling for invalid Julian dates and leap year considerations
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*> Performance optimized CENTURY-DAY processing
DATA DIVISION.
WORKING-STORAGE SECTION.
01  PERFORMANCE-OPTIMIZATION.
    05  BATCH-PROCESS-SIZE PIC 9(6) VALUE 10000.
    05  CACHE-SIZE         PIC 9(4) VALUE 1000.
    05  PIVOT-CACHE        OCCURS 1000 TIMES.
        10  PC-INPUT       PIC 9(5).
        10  PC-PIVOT       PIC 99.
        10  PC-RESULT      PIC 9(7).
        10  PC-VALID       PIC X VALUE 'N'.
    05  CACHE-HITS         PIC 9(8) VALUE ZERO.
    05  CACHE-MISSES       PIC 9(8) VALUE ZERO.
 
01  VALIDATION-CONTROLS.
    05  MAX-JULIAN-DAYS    PIC 9(3) VALUE 366.
    05  MIN-JULIAN-DAYS    PIC 9(3) VALUE 001.
    05  VALIDATION-ERRORS  PIC 9(6) VALUE ZERO.
 
PROCEDURE DIVISION.
OPTIMIZED-CENTURY-DAY-PROCESSING.
    PERFORM INITIALIZE-CACHE
    PERFORM BATCH-PROCESS-JULIAN-DATES
    PERFORM DISPLAY-PERFORMANCE-METRICS.
 
INITIALIZE-CACHE.
    PERFORM VARYING WS-I FROM 1 BY 1 UNTIL WS-I > CACHE-SIZE
        MOVE 'N' TO PC-VALID(WS-I)
        MOVE ZERO TO PC-INPUT(WS-I) PC-RESULT(WS-I)
    END-PERFORM.
 
BATCH-PROCESS-JULIAN-DATES.
    PERFORM VARYING WS-RECORD FROM 1 BY 1 
        UNTIL WS-RECORD > BATCH-PROCESS-SIZE
        
        *> Read julian date from input
        PERFORM READ-JULIAN-DATE-RECORD
        
        *> Validate before processing
        PERFORM VALIDATE-JULIAN-DATE
        
        IF JULIAN-DATE-VALID
            PERFORM CACHED-CENTURY-DAY-CONVERSION
        ELSE
            ADD 1 TO VALIDATION-ERRORS
            PERFORM LOG-VALIDATION-ERROR
        END-IF
    END-PERFORM.
 
VALIDATE-JULIAN-DATE.
    MOVE 'Y' TO JULIAN-DATE-VALID
    
    *> Extract day portion
    COMPUTE WS-DAY-PORTION = FUNCTION MOD(WS-INPUT-JULIAN, 1000)
    
    *> Check day range
    IF WS-DAY-PORTION < MIN-JULIAN-DAYS OR 
       WS-DAY-PORTION > MAX-JULIAN-DAYS
        MOVE 'N' TO JULIAN-DATE-VALID
    END-IF
    
    *> Check for leap year validity
    IF WS-DAY-PORTION = 366
        COMPUTE WS-YEAR-PORTION = WS-INPUT-JULIAN / 1000
        PERFORM CHECK-LEAP-YEAR-VALIDITY
        IF NOT LEAP-YEAR-VALID
            MOVE 'N' TO JULIAN-DATE-VALID
        END-IF
    END-IF.
 
CACHED-CENTURY-DAY-CONVERSION.
    *> Check cache first
    PERFORM CHECK-CONVERSION-CACHE
    
    IF CACHE-HIT
        ADD 1 TO CACHE-HITS
        MOVE PC-RESULT(WS-CACHE-INDEX) TO WS-OUTPUT-JULIAN
    ELSE
        ADD 1 TO CACHE-MISSES
        COMPUTE WS-OUTPUT-JULIAN = 
            FUNCTION CENTURY-DAY(WS-INPUT-JULIAN, WS-PIVOT-YEAR)
        PERFORM UPDATE-CONVERSION-CACHE
    END-IF.
 
CHECK-CONVERSION-CACHE.
    MOVE 'N' TO CACHE-HIT
    
    PERFORM VARYING WS-I FROM 1 BY 1 UNTIL WS-I > CACHE-SIZE
        IF PC-VALID(WS-I) = 'Y' AND
           PC-INPUT(WS-I) = WS-INPUT-JULIAN AND
           PC-PIVOT(WS-I) = WS-PIVOT-YEAR
            MOVE 'Y' TO CACHE-HIT
            MOVE WS-I TO WS-CACHE-INDEX
            EXIT PERFORM
        END-IF
    END-PERFORM.
 
UPDATE-CONVERSION-CACHE.
    *> Find empty slot or least recently used
    PERFORM FIND-CACHE-SLOT
    
    MOVE WS-INPUT-JULIAN TO PC-INPUT(WS-CACHE-SLOT)
    MOVE WS-PIVOT-YEAR TO PC-PIVOT(WS-CACHE-SLOT)
    MOVE WS-OUTPUT-JULIAN TO PC-RESULT(WS-CACHE-SLOT)
    MOVE 'Y' TO PC-VALID(WS-CACHE-SLOT).
 
DISPLAY-PERFORMANCE-METRICS.
    DISPLAY "=== CENTURY-DAY Performance Metrics ==="
    DISPLAY "Records Processed: " BATCH-PROCESS-SIZE
    DISPLAY "Validation Errors: " VALIDATION-ERRORS
    DISPLAY "Cache Hits: " CACHE-HITS
    DISPLAY "Cache Misses: " CACHE-MISSES
    
    IF (CACHE-HITS + CACHE-MISSES) > ZERO
        COMPUTE WS-HIT-RATIO = 
            (CACHE-HITS / (CACHE-HITS + CACHE-MISSES)) * 100
        DISPLAY "Cache Hit Ratio: " WS-HIT-RATIO "%"
    END-IF.

Comprehensive FAQ

Q: What is the difference between CENTURY-DAY and regular date functions?

CENTURY-DAY specifically handles Julian date formats (YYDDD to YYYYDDD conversion) while regular date functions typically work with Gregorian calendar formats. Julian dates are used in scientific and astronomical applications where day-of-year precision is more important than month/day notation.

Q: How do I choose the right pivot year for century windowing?

The pivot year should be chosen based on your data characteristics. For historical data, use a pivot that puts most dates in the past century. For future-oriented data, use a pivot that favors the current/future century. Common choices are 30 (1930-2029), 50 (1950-2049), or 80 (1980-2079).

Q: Can CENTURY-DAY handle leap years correctly?

Yes, CENTURY-DAY correctly processes Julian day 366 for leap years. However, you should validate that day 366 is only used in actual leap years to prevent invalid date scenarios. The function will accept day 366 but the resulting date may be invalid if the year is not a leap year.

Q: What happens if I pass an invalid Julian date to CENTURY-DAY?

CENTURY-DAY may accept invalid Julian dates (like day 000 or day 400) and produce a result, but the resulting date will be meaningless. Always validate Julian dates before conversion: days 001-365 for regular years, 001-366 for leap years.

Q: How does CENTURY-DAY performance compare to other date functions?

CENTURY-DAY is generally efficient as it performs simple arithmetic operations. Performance can be optimized by caching results for repeated conversions with the same pivot year, validating inputs before conversion, and processing dates in batches when possible.

Q: Can I use CENTURY-DAY for astronomical calculations?

Yes, CENTURY-DAY is well-suited for astronomical applications where Julian dates are common. However, for precise astronomical calculations, you may need to convert to Julian Day Numbers (JDN) or Modified Julian Day (MJD) formats, which require additional calculations beyond CENTURY-DAY.

Interactive Quiz

Test Your CENTURY-DAY Knowledge

1. What does CENTURY-DAY convert?

2. What is the purpose of the pivot year in CENTURY-DAY?

3. What is the valid range for Julian day numbers?

Answers: 1-B, 2-B, 3-B

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WORKING-STORAGE SECTION.
01  DATE-ARITHMETIC-WORKSPACE.
    05  START-JULIAN       PIC 9(5).
    05  END-JULIAN         PIC 9(5).
    05  START-CENTURY      PIC 9(7).
    05  END-CENTURY        PIC 9(7).
    05  DURATION-DAYS      PIC 9(5).
    05  DURATION-WEEKS     PIC 9(3)V99.
    05  DURATION-YEARS     PIC 9(2)V999.
 
01  BUSINESS-CALENDAR.
    05  PROJECT-START      PIC 9(5) VALUE 23001.  *> Jan 1, 2023
    05  PROJECT-PHASES     OCCURS 10 TIMES.
        10  PHASE-DURATION PIC 9(3).
        10  PHASE-END      PIC 9(7).
    05  TOTAL-PROJECT-DAYS PIC 9(4).
 
01  SEASONAL-CALCULATIONS.
    05  SPRING-EQUINOX     PIC 9(5) VALUE 23080.  *> Day 80, 2023
    05  SUMMER-SOLSTICE    PIC 9(5) VALUE 23172.  *> Day 172, 2023
    05  AUTUMN-EQUINOX     PIC 9(5) VALUE 23266.  *> Day 266, 2023
    05  WINTER-SOLSTICE    PIC 9(5) VALUE 23355.  *> Day 355, 2023
    05  SEASON-LENGTH      PIC 9(3).
 
PROCEDURE DIVISION.
DATE-ARITHMETIC-OPERATIONS.
    PERFORM PROJECT-TIMELINE-CALCULATION
    PERFORM SEASONAL-ANALYSIS
    PERFORM BUSINESS-DAY-CALCULATIONS.
 
PROJECT-TIMELINE-CALCULATION.
    *> Calculate project phase durations
    MOVE 23001 TO START-JULIAN  *> Project start
    COMPUTE START-CENTURY = FUNCTION CENTURY-DAY(START-JULIAN, 30)
    
    *> Initialize phase durations
    MOVE 30 TO PHASE-DURATION(1)   *> Phase 1: 30 days
    MOVE 45 TO PHASE-DURATION(2)   *> Phase 2: 45 days
    MOVE 60 TO PHASE-DURATION(3)   *> Phase 3: 60 days
    MOVE 20 TO PHASE-DURATION(4)   *> Phase 4: 20 days
    
    MOVE START-CENTURY TO WS-CURRENT-JULIAN
    MOVE ZERO TO TOTAL-PROJECT-DAYS
    
    PERFORM VARYING WS-PHASE FROM 1 BY 1
        UNTIL WS-PHASE > 4
        
        ADD PHASE-DURATION(WS-PHASE) TO WS-CURRENT-JULIAN
        MOVE WS-CURRENT-JULIAN TO PHASE-END(WS-PHASE)
        ADD PHASE-DURATION(WS-PHASE) TO TOTAL-PROJECT-DAYS
        
        DISPLAY "Phase " WS-PHASE " ends on: " PHASE-END(WS-PHASE)
    END-PERFORM
    
    DISPLAY "Total project duration: " TOTAL-PROJECT-DAYS " days".
 
SEASONAL-ANALYSIS.
    *> Calculate seasonal durations
    COMPUTE START-CENTURY = FUNCTION CENTURY-DAY(SPRING-EQUINOX, 30)
    COMPUTE END-CENTURY = FUNCTION CENTURY-DAY(SUMMER-SOLSTICE, 30)
    COMPUTE SEASON-LENGTH = END-CENTURY - START-CENTURY
    DISPLAY "Spring duration: " SEASON-LENGTH " days"
    
    COMPUTE START-CENTURY = FUNCTION CENTURY-DAY(SUMMER-SOLSTICE, 30)
    COMPUTE END-CENTURY = FUNCTION CENTURY-DAY(AUTUMN-EQUINOX, 30)
    COMPUTE SEASON-LENGTH = END-CENTURY - START-CENTURY
    DISPLAY "Summer duration: " SEASON-LENGTH " days"
    
    COMPUTE START-CENTURY = FUNCTION CENTURY-DAY(AUTUMN-EQUINOX, 30)
    COMPUTE END-CENTURY = FUNCTION CENTURY-DAY(WINTER-SOLSTICE, 30)
    COMPUTE SEASON-LENGTH = END-CENTURY - START-CENTURY
    DISPLAY "Autumn duration: " SEASON-LENGTH " days".
 
BUSINESS-DAY-CALCULATIONS.
    *> Calculate business days between dates
    MOVE 23100 TO START-JULIAN     *> Day 100
    MOVE 23200 TO END-JULIAN       *> Day 200
    
    COMPUTE START-CENTURY = FUNCTION CENTURY-DAY(START-JULIAN, 30)
    COMPUTE END-CENTURY = FUNCTION CENTURY-DAY(END-JULIAN, 30)
    COMPUTE DURATION-DAYS = END-CENTURY - START-CENTURY
    
    *> Convert to weeks and years
    COMPUTE DURATION-WEEKS = DURATION-DAYS / 7
    COMPUTE DURATION-YEARS = DURATION-DAYS / 365.25
    
    DISPLAY "Duration Analysis:"
    DISPLAY "Days: " DURATION-DAYS
    DISPLAY "Weeks: " DURATION-WEEKS
    DISPLAY "Years: " DURATION-YEARS.

Leap Year and Calendar Validation

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*> Leap year processing and calendar validation
DATA DIVISION.
WORKING-STORAGE SECTION.
01  LEAP-YEAR-PROCESSING.
    05  TEST-YEAR          PIC 9(4).
    05  IS-LEAP-YEAR       PIC X VALUE 'N'.
        88  LEAP-YEAR      VALUE 'Y'.
    05  MAX-DAYS           PIC 9(3).
    05  VALIDATION-STATUS  PIC X VALUE 'Y'.
        88  VALID-DATE     VALUE 'Y'.
 
01  CALENDAR-VALIDATION.
    05  INPUT-JULIAN       PIC 9(5).
    05  EXTRACTED-YEAR     PIC 9(4).
    05  EXTRACTED-DAY      PIC 9(3).
    05  ERROR-MESSAGE      PIC X(50).
 
01  DATE-CONVERSION-TABLE.
    05  MONTH-DAYS         OCCURS 12 TIMES PIC 9(2).
    05  CUMULATIVE-DAYS    OCCURS 12 TIMES PIC 9(3).
 
PROCEDURE DIVISION.
LEAP-YEAR-AND-VALIDATION.
    PERFORM INITIALIZE-MONTH-TABLE
    PERFORM VALIDATE-JULIAN-DATES
    PERFORM LEAP-YEAR-ANALYSIS.
 
INITIALIZE-MONTH-TABLE.
    *> Initialize days per month
    MOVE 31 TO MONTH-DAYS(1)   *> January
    MOVE 28 TO MONTH-DAYS(2)   *> February (non-leap)
    MOVE 31 TO MONTH-DAYS(3)   *> March
    MOVE 30 TO MONTH-DAYS(4)   *> April
    MOVE 31 TO MONTH-DAYS(5)   *> May
    MOVE 30 TO MONTH-DAYS(6)   *> June
    MOVE 31 TO MONTH-DAYS(7)   *> July
    MOVE 31 TO MONTH-DAYS(8)   *> August
    MOVE 30 TO MONTH-DAYS(9)   *> September
    MOVE 31 TO MONTH-DAYS(10)  *> October
    MOVE 30 TO MONTH-DAYS(11)  *> November
    MOVE 31 TO MONTH-DAYS(12)  *> December
    
    *> Calculate cumulative days
    MOVE 31 TO CUMULATIVE-DAYS(1)
    PERFORM VARYING WS-MONTH FROM 2 BY 1
        UNTIL WS-MONTH > 12
        COMPUTE CUMULATIVE-DAYS(WS-MONTH) = 
            CUMULATIVE-DAYS(WS-MONTH - 1) + MONTH-DAYS(WS-MONTH)
    END-PERFORM.
 
VALIDATE-JULIAN-DATES.
    *> Test various Julian dates for validity
    PERFORM VARYING WS-TEST-JULIAN FROM 00001 BY 50
        UNTIL WS-TEST-JULIAN > 99366
        
        MOVE WS-TEST-JULIAN TO INPUT-JULIAN
        PERFORM VALIDATE-SINGLE-JULIAN-DATE
        
        IF NOT VALID-DATE
            DISPLAY "Invalid Julian date: " INPUT-JULIAN
            DISPLAY "Error: " ERROR-MESSAGE
        END-IF
    END-PERFORM.
 
VALIDATE-SINGLE-JULIAN-DATE.
    MOVE 'Y' TO VALIDATION-STATUS
    MOVE SPACES TO ERROR-MESSAGE
    
    *> Convert to century format for year extraction
    COMPUTE WS-CENTURY-JULIAN = FUNCTION CENTURY-DAY(INPUT-JULIAN, 30)
    COMPUTE EXTRACTED-YEAR = WS-CENTURY-JULIAN / 1000
    COMPUTE EXTRACTED-DAY = FUNCTION MOD(WS-CENTURY-JULIAN, 1000)
    
    *> Check if day is within valid range
    IF EXTRACTED-DAY < 1 OR EXTRACTED-DAY > 366
        MOVE 'N' TO VALIDATION-STATUS
        MOVE "Day of year out of range (1-366)" TO ERROR-MESSAGE
        EXIT PARAGRAPH
    END-IF
    
    *> Check leap year constraints
    MOVE EXTRACTED-YEAR TO TEST-YEAR
    PERFORM CHECK-LEAP-YEAR
    
    IF LEAP-YEAR
        MOVE 366 TO MAX-DAYS
    ELSE
        MOVE 365 TO MAX-DAYS
    END-IF
    
    IF EXTRACTED-DAY > MAX-DAYS
        MOVE 'N' TO VALIDATION-STATUS
        STRING "Day " EXTRACTED-DAY " invalid for year " 
               EXTRACTED-YEAR " (max: " MAX-DAYS ")"
            DELIMITED BY SIZE INTO ERROR-MESSAGE
        END-STRING
    END-IF.
 
CHECK-LEAP-YEAR.
    MOVE 'N' TO IS-LEAP-YEAR
    
    *> Leap year algorithm
    IF FUNCTION MOD(TEST-YEAR, 4) = 0
        IF FUNCTION MOD(TEST-YEAR, 100) = 0
            IF FUNCTION MOD(TEST-YEAR, 400) = 0
                MOVE 'Y' TO IS-LEAP-YEAR
            END-IF
        ELSE
            MOVE 'Y' TO IS-LEAP-YEAR
        END-IF
    END-IF.
 
LEAP-YEAR-ANALYSIS.
    DISPLAY "Leap Year Analysis for Recent Years:"
    PERFORM VARYING TEST-YEAR FROM 1996 BY 4
        UNTIL TEST-YEAR > 2024
        
        PERFORM CHECK-LEAP-YEAR
        IF LEAP-YEAR
            DISPLAY "Year " TEST-YEAR " is a leap year"
        ELSE
            DISPLAY "Year " TEST-YEAR " is not a leap year"
        END-IF
    END-PERFORM.

Best Practices

CENTURY-DAY Guidelines:

  • Validate Julian dates against leap year constraints
  • Use appropriate pivot years for your application context
  • Handle edge cases like day 366 in non-leap years
  • Document century windowing rules clearly
  • Test with boundary dates (Jan 1, Dec 31, Feb 29)
  • Consider astronomical vs. calendar year differences
  • Implement proper error handling for invalid Julian dates

FAQ

Q: How do I handle day 366 in non-leap years?

CENTURY-DAY will accept day 366, but you should validate against the actual year to ensure it's a leap year. Implement additional validation logic to catch this error.

Q: What's the difference between Julian Day and Julian Date?

Julian Date (YYDDD) is the day of the year format used in COBOL. Julian Day Number is an astronomical concept counting days since January 1, 4713 BCE.

Q: Can CENTURY-DAY handle dates before 1900?

Yes, depending on your pivot year setting. With appropriate windowing, you can handle dates in the 1800s or other centuries.