Roughly, RBDigitalAPI.populate_all_catalog would be run if there were no timestamp, otherwise, RBDigitalAPI.populate_delta would be run basically from the date of the last timestamp (or maybe a day earlier just to be safe).

When it's run with no start date, it:

• perform equivalent of RBDigitalAPI.populate_all_catalog
• set .finish

When it runs with a start date, it uses .finish or perhaps the next earlier snapshot as the begin date and today as the end date (adjusted for availability) to calculate the delta.

• perform equivalent of RBDigitalAPI.populate_delta from .finish to today
• set .finish