CALDAT

Syntax | Arguments | Keywords | Examples | Version History | See Also

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.

Note
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. The CALDAT procedure reflects the adjustment for dates after October 4, 1582. See the example below for an illustration.

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.

Syntax

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

Arguments

Julian

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

Note
Julian values must be in the range -1095 to 1827933925, which corresponds to calendar dates 1 Jan 4716 B.C.E. and 31 Dec 5000000, respectively.

Note
Julian Day Numbers 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 longword integer or longword 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

None.

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 


Warning

You should be aware of 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 Day Numbers 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 something like:

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 

Version History

See Also

BIN_DATE, JULDAY, SYSTIME