ST produces a line called the TIMEKEEPER series. They are basically a standard 28-pin SRAM with battery backup and an RTC mapped into the last few bytes of memory. Not as sexy or inexpensive as some of the I2C solutions but REALLY easy to use. Very stable as well. Two years ago I used one in a farm irrigation control project here in Texas. It easily reaches 120+degrees F in that box in the summer but the clock only looses a few seconds each season.