Shell Programming and Scripting

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#return seconds since `00:00:00 1970-01-01 UTC' (a GNU extension)
######################################
date2stamp () {
    date --utc --date "$1" +%s
}
######################################
# finds number of days between two times
# dateDiff -d "23:59:59" "16:15:07"
# number of hours between two times
# dateDiff -h "23:59:59" "16:15:07"
# number of minutes between two times
# dateDiff -m "23:59:59" "16:15:07"
# number of seconds between two times
# dateDiff -s "23:59:59" "16:15:07"
######################################
dateDiff (){
    case $1 in
        -s)   sec=1;      shift;;
        -m)   sec=60;     shift;;
        -h)   sec=3600;   shift;;
        -d)   sec=86400;  shift;;
        *)    sec=86400;;
    esac
    dte1=$(date2stamp $1)
    dte2=$(date2stamp $2)
    diffSec=$((dte2-dte1))
    if ((diffSec < 0)); then abs=-1; else abs=1; fi
    return $((diffSec/sec*abs))
}

dateDiff -h "23:59:59" `date '+%T'`