That's what they where designed for and if d day had stalled they would have been used in the rhur.
There is also strong evidence that Stalin wanted to get to berlin first to grab some of the German nuclear materiel in Berlin
I have heard discussion that those in control were worried about dropping a nuke over Germany for fear that, should it not explode, it would wind up in the hands of enemy scientists. Japan was not considered capable of repaiting and turning around such a device. Some artillery and anti-aircraft shells during WWII were not permitted to be used over land for similar reasons.
The timelines, the actual dates that the bombs became available, are a little secondary imho as this was a multi-year developmental program. I still doubt they would have dropped a bomb on germany for a variety of reasons both technological and sociological.
We don't know whether atomic bombs would have been used on, say, Berlin or the Ruhr area (maybe even tactically, to speed up the allied advance in the west), but I think the USA would have used one, if only to send a message to the soviets.