The Julian date "2451044.000000" means, for example,
"August 8th 1998, high noon"
and "2451044.25" means
"August 8th 1998, high noon plus a quarter day", i.e. 18:00.
There are two C programs in the auxiliary directory in the xmgr/grace installation directory. You can take them as an example for converting between Julian date and the human-readable format.
Using doubles as the representation means that there is no problem with the internal representation of date and time, the initial cause of trouble with the year 2000.
However, there are two other issues to be taken into account as for the year 2000 problem. The first one is output. Usually it does not make sense to use the Julian date for tick labels, so one uses some of the more appropriate formats, such as "DD-MM-YY". Although this notation seems to suggest that the year is always 2 digits, it is not. For Julian dates that are greater than the start of the new millenium, the year will have 4 digits and be correct.
The other issue is input format. There is something in the FAQ telling
you that you can use data input files where the date is not Julian but
something such as YY-MM-DD by prepending a line
@format YYMMDD
and so forth. The question is what happens if the year in the data file
has four digits. The answer is: here you should always have four
digits, since 99 is interpreted as 99 AC, not 1999 AC. In other words,
here is no year 2000 problem as well.
To sum it up: I cannot see any problem with xmgr related to the year
2000.
If somebody else does, correct me.
As usual, when it comes to a free software, there are NO warranties of any kind. See the COPYRIGHT statement.
Dr. Henrik Seidel, http://www.mpimg-berlin-dahlem.mpg.de/~seidel/