I'm just posting this out of prior experience from working at GameStop (Though GameStop.com is an entire different beast)
When a promotion says ends 2/5 or whatever, it goes till the end of the day. Even in store, that technically meant if someone broke in and purchased a game during B2G1, it would still ring the 3rd up free.
With that being said, the website is different at times. I'd at least shoot an e-mail to them? They might retroactively let you place it for that.