Yes, Merlin uses the date and I agree that it most likely uses the date (along with location) to reference migration patterns. But usually I do not have to input the date, as Merlin seems to take it from the photo information. I do not normally tag my photos with GPS location. If I did, perhaps Merlin could read that too.Merlin's "Photo ID" also asks for the date. I assume it is checking against migration times that could influence its suggestions.
I'm not sure whether Merlin or iBird Pro is more accurate on Photo ID. I often use both on difficult subjects. As far as I can tell, iBird Pro does not take location into account, which often may be valuable information. Merlin is also broader in coverage. If I recall, iBird Pro is limited to North America. Not sure if I will continue with iBird Pro once my one-time purchase lapses and a subscription is required.