How to get elapsed time from two "text based" dates

Question

I would like to know if there is a way to calculate the difference between two times in bash. I have two fields that are extracted from a log file:

Start time: Feb 12 10:02:10 End time: Feb 12 10:53:15

What I need is a manual way to get the elapsed time between these 2 dates. Since the time values are "text based", and not from a date generating command, I don't know if there's a way to do it. The times are 24 hour based, so hopefully that'll make times spanning midnight easier to handle. Any suggestions will be greatly appreciated. Peter V.

Answer 1

You could use some evil trickery:

date -d "Feb 12 10:02:10" +%s

will return the number of seconds since the epoch for that date. Combining this with the same command (for the other date) and running through bc:

echo "$(date -d 'Feb 12 10:53:15' +%s) - $(date -d 'Feb 12 10:02:10' +%s)" | bc

or the shell's arithmetic expansion:

echo "$(( $(date -d 'Feb 12 10:53:15' +%s) - $(date -d 'Feb 12 10:02:10' +%s) ))"

will give you

3065

the number of seconds between the times. You could probably parse this out with awk somehow if you needed to run it more than once.

Answer 2

If you don't mind an external helper utility, have a look at my dateutils. Above task translates to:

ddiff -i '%b %d %H:%M:%S' 'Feb 12 10:02:10' 'Feb 12 10:53:15'
=> 
  3065s

where -i is used to describe the input date format.

Answer 3

Another solution without bc

1. Convert the dates to Unix time (also known as POSIX time or UNIX Epoch time)

$ date1=$(date -d "Feb 12 10:02:10" +%s)
$ date2=$(date -d "Feb 12 10:53:15" +%s)

$ printf '%s\n' "$date1" "$date2"
1518447730
1518450795
$ 

2. Calculate the difference between the two dates

$ datediff="$((date2-date1))"

3. You'll get the answer expressed in seconds

$ echo "$datediff"
3065
$

4. Convert it to hours, minutes and seconds format

$ printf '%02dh:%02dm:%02ds\n' $(($datediff/3600)) $(($datediff%3600/60)) $(($datediff%60))
00h:51m:05s

or

$ TZ=UTC date '+%H:%M:%S' -d"@$datediff"
00:51:05

Be alert to the possibility that the result is ≥ 86400 (i.e., 24 hours) and adjust the format accordingly.