timebetween.sh

#!/usr/bin/env bash

output() {
    printf "%s %s\n" "$@"
}

STARTDATE="$1"
ENDDATE="$2"

STARTSECONDS=$(date +%s -d"${STARTDATE}")
ENDSECONDS=$(date +%s -d"${ENDDATE}")

DIFFSECONDS=$(( ENDSECONDS - STARTSECONDS ))

UNIT="years"
UNITPRINT="year(s)"

if (( DIFFSECONDS < 60 )); then
    output "$DIFFSECONDS" "second(s)"
    exit 0

elif (( DIFFSECONDS < 3600 )); then
    UNIT="minutes"
    UNITPRINT="minute(s)"

elif (( DIFFSECONDS < 86400 )); then
    UNIT="hours"
    UNITPRINT="hour(s)"

elif (( DIFFSECONDS < 604800 )); then
    UNIT="days"
    UNITPRINT="day(s)"

elif (( DIFFSECONDS < 31536000 )); then
    UNIT="weeks"
    UNITPRINT="week(s)"
fi

DIFFTIME=$(units "${DIFFSECONDS}seconds" "${UNIT}" | awk '/*/ {print $2}')
output "$DIFFTIME" "${UNITPRINT}"