Shell Programming and Scripting

t1=11:48:30
t2=13:13:48

t=$(date -d "20121012 11:48:30" +%s)
t1=$(date -d "20121012 13:13:48" +%s)
diff=$(expr $t1 - $t)
echo $diff

#!/bin/ksh
# secdiff -- calculate seconds between two timestamps
# secdiff [start_time [end_time]]
# Calculate the number of seconds between start_time and end_time.  Both
# timestamps must be in the form:
#               HH:MM:SS
# with HH, MM, and SS each being two digit decimal strings representing the
# hours, minutes, and seconds, respectively, since midnight using 24 hour clock
# notation.  If end_time is earlier in the day than start_time, end_time is
# assumed to be on the day after start_time.  This utility does not work
# correctly if there is a shift to or from daylight savings time between
# start_time and end_time.

# Set t1 and t2 to $1 and $2 with default values from the given test case.
t1=${1:-11:48:30}
t2=${2:-13:13:48}
t1s=${t1##*:}   # Extract the seconds from $t1
t1h=${t1%%:*}   # Extract the hours from $t1
t1m=${t1%:*}    # Extract the minutes from $t1 (step 1)
t1m=${t1m#*:}   # Extract the minutes from $t1 (step 2)
t1ssm=$((t1h * 3600 + t1m * 60 + t1s)) # Calculate $t1 seconds since midnight
t2s=${t2##*:}   # Perform the same operations on $t2
t2h=${t2%%:*}
t2m=${t2%:*}
t2m=${t2m#*:}
t2ssm=$((t2h * 3600 + t2m * 60 + t2s))
if [ $t1ssm -gt $t2ssm ] # If $t1ssm > $t2ssm, $t2 must be on the next day
then    t2ssm=$((t2ssm + 60 * 60 * 24))
fi
printf "%s is %d seconds after %s\n" $t2 $((t2ssm - t1ssm)) $t1

./secdiff
    or
./secdiff 11:48:30 13:13:48

./secdiff 23:59:58 00:00:02

#!/bin/ksh
t1=${1:-11:48:30}
t2=${2:-13:13:48}
OIFS=$IFS
IFS=': '
set -- $t2 $t1
IFS=$OIFS
diff=$(($1*3600+$2*60+$3 - $4*3600-$5*60-$6))
[ $diff -lt 0 ] && diff=$((diff + 3600 * 24))
printf "%s is %d seconds after %s\n" $t2 $diff $t1