don't try to compute credit/debit beyond the oldest record in the database
Here's how credit/debit works now:
- When the period is set to infinite relative to today, the credit/debit goes as far back in time as one has data.
- When the period is set to anything else relative to today, the credit/debit goes back to the end of the defined period without regard for whether or not one has data back then.
It seems like it should never go farther back in time than one's oldest record.