Field help (ECARDATDEB) 

For Journal entry type variances, this process includes entries that are not closed at the end of the period. The beginning of the period must enable all the non-matched journals to be processed. The period start date is usually a date prior to the start date of the fiscal year.

For Account balance type variances, this process includes balances of accounts managed in currency, in other words on the balance. The period start is the start of the fiscal year.