%[format]j[[+|-]n] references the
current respectively queried Julian day number relative to the actual
system date (≡today). This day number is based on the date 1st
January 4713 BCE —which is the starting day zero of a consecutive day
counting used in astronomical computations— and is known as the Julian
Date (J.D.). The real zero of this date is at 12 o’clock Universal time
(UTC/GMT); the day does not change at midnight, but at noon Universal
time. Here, Gcal does not evaluate the timezone returned by the systems
date function. For that reason, this day number is represented
without a time fraction on the supposition that the day has already changed
at noon. If you do not like the feature that the day displayed has already
changed at noon, you can decrease the resulting Julian day number of that
special text always by one, e.g. ‘%j-1’.
The resource file line ‘0 Julian day %j since 01-Jan-4713 BCE’ will be expanded to
→ ‘Julian day 2437929 since 01-Jan-4713 BCE’, in case you call Gcal with the -c %19620921 option and no command.
%[format]j text is directly trailed by an unsigned
number, this number is always subtracted from the real Julian day number. So
you are able to work with any quantities referenced, e.g.:
The resource file line ‘0 Julian day %j2415021 since 01-Jan-1900’ will be expanded to
→ ‘Julian day 4 since 01-Jan-1900’, in case you call Gcal with the -c %19000105 option and no command.