I would like to convert a date/times to the Julian Date (number of days since Jan 1 4713 BCE, not the YY-ddd form where ddd is the day number of the current year). This doesn't seem to be built-in to gnudate, but I suspect it's pretty easy with the right incantation of date and bc.
I would prefer to be able to do this from the shell prompt or via a bash function rather than having to install some extra packages. Gnu-date and standard tools like sed awk bc
OLD_JULIAN_VAR=$(date -u -d 1840-12-31 +%s)
TODAY_DATE=`date --date="$odate" +"%Y-%m-%d"`
TODAY_DATE_VAR=`date -u -d "$TODAY_DATE" +"%s"`
export JULIAN_DATE=$((((TODAY_DATE_VAR - OLD_JULIAN_VAR))/86400))
echo $JULIAN_DATE
the mathematically
[(date in sec)-(1840-12-31 in sec)]/86400
If you're only interested in dates that GNU date supports, you can make it spit out the number of seconds since 1970-01-01. Unix time uses a fixed number of seconds per day (leap seconds are smoothed out), so you can convert that number of seconds into a number of days simply by dividing by 86400. To convert that into the Julian day number, add the requisite fixed offset.
echo $(($(TZ=GMT+12 date +%s -d "now") / 86400 + 2440587))
