AUK results are normally recorded with the start date, whatever the distance or finishing date.
Indeed, and as wilkyboy says it would only matter at end of season - unless you were doing something like an SRRTY.
On the other hand, if you want it to count for the later month for RRTY, and you can, if necessary, demonstrate that you rode at least 200km of the ride in that later month, then Mike Wigley has said
1 that he will be sympathetic.
1: elsewhere, and in a different thread.