dd/mm/yyfor 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:ssssThe text of the proposed standard is available.
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 RVSAO. Decoding of ISO 8601 dates was added to the date decoding subroutine imgdate() in Util/imgpar.x in September 1997. All versions of RVSAO after the first 2.0 beta release in January 1998 contained the fix.