It's the activity date, not the posting date, that counts. That said, I've just realised that both of the accounts I am managing have posted with an e-rewards activity date 3 or 4 days later than the actual transfer, so you might have to get DM to correct the date if you can't initiate the transfer until the last few days of GS.

Edited to say:

Both the above quoted posters are referring to "activity lag" (the difference between the actual and recorded date of activity), not suggesting that the hit needs to post by the end of GS. Sorry for my initial misunderstanding.