Add a 'before' date to Export in the same way as Purge does

[BleachDev]

• Currently Purge allows a date e.g. purge records before 25 October
• Currently Export will always export right up until the present moment
• Therefore unless Purging and Exporting at a time of day outside of any experiment time windows there is always duplication of records for the present day.
• If Export allows a date e.g. export records before 25 October then exporting and purging can be synchronised by excluding the current day from both.