SAOimage and Y2K

Dates in FITS data files

The original FITS standard for astronomical data files uses the format

dd/mm/yy

for dates used as values for such header keywords as DATE and DATE-OBS. The 2-digit year is assumed to have 1900 added to it to get the actual year of observation. There is thus a problem when the actual year is 2000 or greater.

Coordinate-related dates, such as EPOCH, which is used as the equinox of the coordinate system in the original standard, and EQUINOX, which is coming to be used to designate the date of the equinox of the coordinate system, are usually given as a fraction of a year, such as 2000.0 or 1950.0. This use of traditional astronomical notation will create no new problems in the year 2000.

After some discussion, the world-wide community of FITS users decided in 1997 to use the the ISO 8601 format for values of DATE-* keywords. In this standard, the date and time are given as

yyyy-mm-ddThh:mm:ssss

The text of the proposed standard is available.

Dates in SAOimage

The only time SAOimage uses dates is when the date of observation is used in conversion between coordinate types, as in B1950 to J2000, where conversion at a specific epoch makes the conversion reversible to at least milliarcseconds instead of tenths of arcseconds without the epoch.

Internally, 4-digit years have ALWAYS been used, with 1900 added to 2-digit years on input. Nonstandard 4-digit years in date strings (dd/mm/yyyy as well as dd/mm/yy) have always been allowed in SAOimage. Decoding of ISO 8601 dates was added to the date decoding subroutine hgetdate() in late July of 1997; the next version of SAOimage, 1.21, which was released in January 1998, contained the fix. Date decoding is complicated by the fact that there are significant numbers of FITS files around with dates written as dd-mm-yy. SAOimage assumes that format instead of ISO 8601 if dd in the ISO format is greater than 31.