How do I create a date time parameter that allows the data to refresh from when the report was last run to the current time?

I am creating a sales report that will be migrated to Crystal Server so that it can be emailed automatically twice a day to managers. They want the data to show sales from when the last report was generated to the current time. The report will be scheduled to run at 12pm (lunchtime) and then again at 5.30pm, so I would need the data from 5.30pm to 12 noon, and then 12 noon to 5.30pm.

I can do currentdatetime for the end date, but how do I get a similar formula which shows when the report was last run?

Thanks.