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.