Previous versions used start_date as a hash key. This version introduces a
deprecation by renaming to start_time for consistency: see
issue #102. The old name will continue to be recognized when
reading seralized schedules from previous versions, and we will default to
exporting serialized schedules with both names. You can disable this
duplication by setting IceCube.compatibility = 12 after your downstream code
is updated to look for start_time. Watch for deprecation notices in your log.