Critical Description
When new facts are added and are populated with data from some time onwards, having start time property on the facts should help picking eligible facts for given range.
Without the property, if a light fact is added from a later time and with lightest fact first flag on, the queries cannot be answered from older range. This causes queries to fail.
Similarly we can add end time as well for the fact.
Also, there could be some facts on which data is valid from relative start time. For example : only 90 days of data is valid. Accepting relative start time will allow, even if drop partitions is not uptodate, query can pick proper facts.