Date Calculation – Quickly Calculate Date Differences

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.

I had to do this in the past with brute force parsing and calculation in shell script.

Doing it manually in shell script is highly error-prone and slow. You need to account for days per month, leap years, time zones. It will fail with different locales and different languages.

I fixed up one of my old scripts, this would probably need to be modified for different shell environments, but there shouldn't be anything that can't be changed to work in any shell.

#!/bin/sh 

datestring="Nov 28 20:27:19 2012 GMT"
today=`date`

function date2epoch {

month=$1
day=$2
time=$3
year=$4
zone=$5

# assume 365 day years
epochtime=$(( (year-1970) * 365 * 24 * 60 * 60 ))

# adjust for leap days
i=1970
while [[ $i -lt $4 ]]
do
    if [[ 0 -eq $(( i % 400 )) ]]
    then
        #echo $i is a leap year
        # divisible by 400 is a leap year
        epochtime=$((epochtime+24*60*60))
    elif [[ 0 -eq $(( i % 100 )) ]]
    then
        #echo $i is not a leap year
        epochtime=$epochtime
    elif [[ 0 -eq $(( i % 4 )) ]]
    then
        #echo $i is a leap year
        # divisible by 4 is a leap year
        epochtime=$((epochtime+24*60*60))
    #   epoch=`expr $epoch + 24 * 60 * 60`
    fi

    i=$((i+1))
done    

dayofyear=0
for imonth in Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
do
    if [[ $month == $imonth ]]
    then
        break
    fi

    case $imonth in
    'Feb')
        if [[ 0 -eq $(( year % 400 )) ]]
        then
            days=29
        elif [[ 0 -eq $(( year % 100 )) ]]
        then
            days=28
        elif [[ 0 -eq $(( year % 4 )) ]]
        then
            days=29
        fi
    ;;

    Jan|Mar|May|Jul|Aug|Oct|Dec) days=31 ;;
    *) days=30 ;;
    esac

    #echo $imonth has $days days
    dayofyear=$((dayofyear + days))
done
## add the day of the month 
dayofyear=$((dayofyear+day))
#echo $dayofyear

########## Add the day fo year (-1) to the epochtime
#(-1, since eg. Jan 1 is not 24 hours into Jan1 )

epochtime=$((epochtime + (dayofyear -1) * 24*60*60))
#echo $epochtime
################## hours, minutes, seconds
OFS=$IFS
IFS=":"
set -- $time
hours=$1
minutes=$2
seconds=$3
epochtime=$((epochtime + (hours * 60 * 60) + (minutes * 60) + seconds))
IFS=$OFS

################## Time zone

case $zone in
'GMT') zonenumber=0
break;;
'EST') zonenumber=-5
break;;
'EDT') zonenumber=-4
break;;
esac

epochtime=$((epochtime + zonenumber * 60 * 60 ))

echo $epochtime
}

result=`date2epoch $datestring`

echo $result

I probably made a mistake somewhere and there may be a better way. Let me know if you find a bug, or a better way.

Once you have the epoch time, you can do some useful calculations... although converting that back into a date without the gnu utils... requires doing the above in reverse...