The CALDAT procedure computes the month, day, year, hour, minute, or second corresponding to a given Julian Date. The inverse of this procedure is JULDAY.

Notes

  • The Julian calendar, established by Julius Caesar in the year 45 BCE, was corrected by Pope Gregory XIII in 1582, excising ten days from the calendar. For dates after 4 October 1582, the calendar specifies that every 4 years is a leap year, except if the year ends in a "00" then it is not a leap year, unless it is also divisible by 400 (in which case it is a leap year).
  • There is no year 0 in the calendar as defined by IDL. Instead, year 1 CE is immediately preceded by year 1 BCE. This means that leap years are offset by 1. For example, -1, -5, -9, -13, etc. are all leap years.
  • The Julian Date is typically used by astronomers. For other research areas, you may need to use the proleptic Gregorian calendar system, especially for dates before 1582. In this case, use the GREG2JUL and JUL2GREG routines instead.
  • The JULDAY function should only be used with CALDAT. Similarly, the GREG2JUL function should only be used with the JUL2GREG procedure. For dates between 1 Jan CE and 15 Oct 1582 the two calendar systems differ by up to 10 days. For dates on or after 15 Oct 1582 the two calendar systems are identical.
  • Julian values must be between -31776 and 1827933925, which correspond to calendar dates of 1 Jan 4800 BCE and 31 Dec 5000000, respectively.
  • This routine is written in the IDL language. Its source code can be found in the file caldat.pro in the lib subdirectory of the IDL distribution.

Examples

In 1582, Pope Gregory XIII adjusted the Julian calendar to correct for its inaccuracy of slightly more than 11 minutes per year. As a result, the day following October 4, 1582 was October 15, 1582. CALDAT follows this convention, as illustrated by the following commands:

CALDAT, 2299160, Month1, Day1, Year1
CALDAT, 2299161, Month2, Day2, Year2
PRINT, Month1, Day1, Year1
PRINT, Month2, Day2, Year2

IDL prints:

10    4    1582
10   15    1582

Be sure to note this discrepancy between the original and revised Julian calendar reckonings if you calculate dates before October 15, 1582.

Be sure to distinguish between Month and Minute when assigning variable names. For example, the following code would cause the Month value to be the same as the Minute value:

;Find date corresponding to Julian day 2529161.36:
CALDAT, 2529161.36, M, D, Y, H, M, S
PRINT, M, D, Y, H, M, S

IDL prints:

0        4        2212           18            0        0.00000000

Moreover, Julian Dates should be maintained as double-precision floating-point data when the numbers are used to determine hours, minutes, and seconds.

So, instead of the previous call to CALDAT, use a "D" following the number to ensure that it is double precision:

CALDAT, 2529161.36D, Month, Day, Year, Hour, Minute, Second
PRINT, Month, Day, Year, Hour, Minute, Second

IDL prints:

7        4        2212           20            38        23.999989

You can also use arrays for the Julian argument:

CALDAT, DINDGEN(4) + 2449587.0D, m, d, y
PRINT, m, d, y

IDL prints:

           8           8           8           8
          22          23          24          25
        1994        1994        1994        1994

Syntax

CALDAT, Julian, Month [, Day [, Year [, Hour [, Minute [, Second]]]]] [, /MODIFIED]

Arguments

Julian

A numeric value or array that specifies the Julian Date (which begins at noon) to be converted to a calendar date.

Note: Julian Dates should be maintained as double-precision floating-point data when the numbers are used to determine hours, minutes, and seconds.

Month

A named variable that, on output, contains a long integer or long-integer array representing the number of the desired month (1 = January, ..., 12 = December).

Day

A named variable that, on output, contains a longword integer or longword integer array representing the number of the day of the month (1-31).

Year

A named variable that, on output, contains a longword integer or longword integer array representing the number of the desired year (e.g., 1994).

Hour

A named variable that, on output, contains a longword integer or longword integer array representing the number of the hour of the day (0-23).

Minute

A named variable that, on output, contains a longword integer or longword integer array representing the number of the minute of the hour (0-59).

Second

A named variable that, on output, contains a double-precision floating-point value or a double-precision floating-point array representing the number of the second of the minute (0-59).

Keywords

MODIFIED

Set this keyword to treat the input values as Modified Julian Dates (equivalent to the regular Julian Date minus 2400000.5).

Note: When converting Modified Julian Dates to and from date/times, be sure to specify the MODIFIED keyword on all calls to JULDAY and CALDAT.

Version History

Pre 4.0

Introduced
8.8.3 Added MODIFIED keyword

See Also

BIN_DATE, GREG2JUL, JUL2GREG, JULDAY, SYSTIME, Date/Time Data