calc-date

usage: ./calc-date <date> +-<delta>|<date>

Returns a date after <delta> is added or substracted to <date> or time difference
between two dates.

date    iso8601 format, e.g. 2015-06-01T09:45:53
delta   time to count from date. Select operation with + or - followed
        by any combination of:
        <n>d for days,
        <n>h for hours,
        <n>m for minutes
        <n>s for seconds.