The usage would be for example to flash a light every 15mins over long periods of time for months or even years an stay relatively accurate over this period of time.
Thanks for this Jack, that looks very useful! I've been doing a bit more research on this RTC stuff (I'm a bit of a noob) :~ and I've found a simular one called DS3234 which is SPI instead?? If someone could kindly enlighten me to what the difference is between I2C and SPI is that would be awesome. Also is this compatible with the attiny chips and the arduino platform.
I2C is also sometimes called TWI (two-wire interface). SPI needs three or four wires usually. Personally I tend to use RTCs with I2C interfaces, but there's nothing wrong with using SPI. The DS3234 is essentially an SPI version of the DS3231 or DS3232, all of which should be available at Mouser or Digikey.