Merge branch '70-valid-dates-datetime' into '2.1-dev'

Alter valid_end and valid_start to use DateTime

See merge request !28
3 jobs for 68-non-expiring in 1 minute and 50 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3740
whats-open-py2.7

00:01:36

passed #3741
whats-open-py3.5

00:01:47

passed #3742
whats-open-py3.6

00:01:50