Board Thread:Show-Related Questions and Answers/@comment-5521796-20130201015632/@comment-4260948-20130204214514

He was hoping that August was Bae; it was the picture of the dagger that clouded his judgement.

It's probable - and most likely - that Rumple discovered when Bae arrived in our world and aimed for that date but arrived 28 years early - thus the whole "the curse will break on Emma's 28th year" stuff.