If you want a RTC and don't want to mess with soldering a replacement battery on, you might want to look for one that has the replaceable button battery on it.
Some trapdoor expansions have a soldered in rechargeable battery I think, but still, if the battery dies, you have to solder a replacement in or a battery holder for a new button style battery.
I'm sure others will have additional suggestions.
Maybe something like this:
http://www.ebay.com/itm/311016244763