Although this field can be modified, it is automatically repopulated when the purge interval is reached by the number of days specified in the "Remove documents not modified in the last X days" setting (File -> Replication -> Settings -> Space Savers). http://www-01.ibm.com/support/docview.wss?uid=swg21208140
In other words, every 30 days (which is the default purge interval), this field will be updated with the date that was 90 days prior to the current date. For example, if the current date is August 9th and it is also the day of the purge interval, the field will be populated with May 9th. Note that, it is not possible to permanently clear this field; it will always be repopulated when the purge interval is reached.