De GNU datum opdracht in vol goodies, maar niet als het gaat om het berekenen van een datum of tijd verschil. Hier is wat ik kwam op na op zoek naar meer dan een oplossing.

Code moet worden zelf uitleggen.

#!/bin/bash

date2stamp () {
    date --utc --date "$1" +%s
}

stamp2date (){
    date --utc --date "1970-01-01 $1 sec" "+%Y-%m-%d %T"
}

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
    echo $((diffSec/sec*abs))
}

# USAGE # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 

# convert a date into a UNIX timestamp
    stamp=$(date2stamp "2006-10-01 15:00")
    echo $stamp

# from timestamp to date
    stamp2date $stamp

# calculate the number of days between 2 dates
    # -s in sec. | -m in min. | -h in hours  | -d in days (default)
    dateDiff -s "2006-10-01" "2006-10-32"
    dateDiff -m "2006-10-01" "2006-10-32"
    dateDiff -h "2006-10-01" "2006-10-32"
    dateDiff -d "2006-10-01" "2006-10-32"
    dateDiff  "2006-10-01" "2006-10-32"

# number of seconds between two times
    dateDiff -s "17:55" "23:15:07"
    dateDiff -m "17:55" "23:15:07"
    dateDiff -h "17:55" "23:15:07"

# number of minutes from now until the end of the year
    dateDiff -m "now" "2006-12-31 24:00:00 CEST"

# Other standard goodies from GNU date not too well documented in the man pages
    # assign a value to the variable dte for the examples below
    dte="2006-10-01 06:55:55"
    echo $dte

    # add 2 days, one hour and 5 sec to any date
    date --date "$dte  2 days 1 hour 5 sec"

    # substract from any date
    date --date "$dte 3 days 5 hours 10 sec ago"
    date --date "$dte -3 days -5 hours -10 sec"

    # or any mix of +/-. What will be the date in 3 months less 5 days
    date --date "now +3 months -5 days"

    # time conversions into ISO-8601 format (RFC-3339 internet recommended format)
    date --date "sun oct 1 5:45:02PM" +%FT%T%z
    date --iso-8601=seconds --date "sun oct 1 5:45:02PM"
    date --iso-8601=minutes

    # time conversions into RFC-822 format
    date --rfc-822 --date "sun oct 1 5:45:02PM"