calculates the time between current time and a given date time.

timeoffset-to.sh

#!/bin/bash
if [[ -z $1 ]]
then
        echo $0 " Usage:"
        echo "  ${0} <date>"
        echo "  Example: ${0} 1990-09-21"
        exit
fi

past_secs=$(date --date ${1} +%s)
now_secs=$(date +%s)
diff=`echo ${past_secs}-${now_secs}|bc`
msecs=`echo ${diff}*1000|bc`
echo ${msecs}