MustParse will panic on an unparsable date string.

Daily will chop up a timespan into 24h intervals.

EveryMinute will chop up a timespan into 60s intervals; https://english.stackexchange.com/questions/3091/weekly-daily-hourly-minutely.

Hourly will chop up a timespan into 60min intervals.

Monthly will chop up a timespan into monthly intervals.

Weekly will chop up a timespan into 7 day intervals.

Date can be used to parse command line args into dates.

Interval groups start and end.