How to get the difference between two dates
It's best if you work with timestamps throughout your code, and then only convert timestamps to human-readable formats for output. If you must handle human-readable dates as input, then you will need something that can parse them.
Using GNU date, for example:
# get the seconds passed since Jan 1, 2010 (localtime) then=$(date -d "2010-01-01 00:00:00" +%s) now=$(date +%s) echo $(($now - $then))
To print a duration as a human-readable value you'll have to do some math:
# some constants minute_secs=60 hour_secs=$((60 * minute_secs)) day_secs=$((24 * hour_secs)) # get total seconds_since=$(($now - $then)) # parse days=$((seconds_since / day_secs)) hours=$((seconds_since % day_secs / hour_secs)) minutes=$((seconds_since % day_secs % hour_secs / minute_secs)) seconds=$((seconds_since % day_secs % hour_secs % minute_secs)) # pretty-print echo "$days days, $hours hours, $minutes minutes and $seconds seconds."
Or, without the verbose labels:
# Bash/ksh ((duration = now - then)) ((days = duration / 86400)) ((duration %= 86400)) ((hours = duration / 3600)) ((duration %= 3600)) ((minutes = duration / 60)) ((seconds = duration % 60)) echo "$days days, $hours hours, $minutes minutes and $seconds seconds."
To convert the timestamp back to a human-readable date, using recent GNU date:
date -d "@$now"
(See FAQ #70 for more about converting Unix timestamps into human-readable dates.)