If Salesforce recalculates the time triggers to a date in the past, Salesforce triggers the associated actions shortly after you save the record.

If a workflow rule has a time trigger set for a time in the past, Salesforce queues the associated time-dependent actions to start executing within one hour. For example, if a workflow rule on opportunities is configured to update a field 7 days before the close date, and you create an opportunity record with the close date set to today, Salesforce starts to process the field update within an hour after you create the opportunity.

Sample Scenario to set Past Time: