Author solution does not count individual days for months and quarters. It'll happily say get_epoch('2019-01-31','m','2019-02-01') === 1 when it's a 1-day period.

Requirement is under-specified. Do we round up, round down, or round to nearest for weeks, months and quarters? How to count months and quarters?

Needs Random Tests

Sample tests

  Test.assertEquals(get_epoch('2019-01-01','m','2019-04-02'),2);