As it always is in electronics and software development, there are many solutions.
I personally use the Maxim DS1388 (http://www.maximinte...cks/DS1388.html). You get an RTC, a watchdog timer, and also 512 bytes of EEPROM. I use it religiously in all my projects.
You can find the driver (DS1388.cs) for it in one of my projects: https://bitbucket.or...tomtypist/ncmp/