date+time->datetime and posix->datetime were computing the JD
in different ways (because it was more efficient that way).
Unfortunately, the different methods of computation don't give
the same results, so now posix->datetime has to do a bit more
work.
Of course, storing the JD in the datetime struct isn't necessary.
Caching it is a performance win in some cases and a loss in others.
date+time->datetimeandposix->datetimewere computing the JD in different ways (because it was more efficient that way). Unfortunately, the different methods of computation don't give the same results, so nowposix->datetimehas to do a bit more work.Of course, storing the JD in the datetime struct isn't necessary. Caching it is a performance win in some cases and a loss in others.