Data transfer schedule.
If the data source does not support a custom schedule, this should be
empty. If it is empty, the default value for the data source will be
The specified times are in UTC.
Examples of valid format:
1st,3rd monday of month 15:30,
every wed,fri of jan,jun 13:15, and
first sunday of quarter 00:00.
See more explanation about the format here:
NOTE: the granularity should be at least 8 hours, or less frequent.