I would like to take a day-trip to Heidelberg while visiting Frankfurt. The time of day I would be able to leave is a little iffy, at this point. If I purchase a ticket via the bahn.de website, is it binding to a specific departure time or can I use it for a later departure if I were to miss the original one?
If you purchase a standard (full) fare ticket, it will be for any train starting with the one you book and will be valid for any later train for (I think) the next month. If you purchase a Savings Fare ticket, it will be for midnight (morning) of that day until 10 AM the following day for any regional train on the ticket, but only for the specific express (IC/ICE/EC) trains on the ticket. Any reservations you get with the ticket will be for that train only.
If you want a standard/full fare ticket, there's no reason to buy online- just use one of the kiosks in the station right before you leave.
The quickest connections between Frankfurt Hauptbahnhof (main station) and Heidelberg are IC/EC trains, and the "savings fare" only saves you 3 euros each way (19 euro vs. 22 euro). The savings fares require you to buy at least three days prior and commit to specific trains. I would prefer the flexibility of a full fare ticket-- you may get bored and want to leave early, or linger over dinner, etc.
Thanks for the replies! Very helpful.