The clock can also be updated if the network sends the time/date info. The setting can be seen in Settings-> Phone Settings -> Date Time -> Automatic date/time update (given for Nokia phone). This is the reason due to which sometimes when we plug in the battery the mobile corrects its clock.
Data in ram is not permanent and there is no way to use that data to determine current time. Secondly the amount of charge held by internal capacitors is too low. As soon as the voltage goes down a certain limit, brown out detection circuitry resets the controller. Thirdly, give a break to the MS designers 🙂 Tomorrow you will say MS should even work when the internal battery dies out OR even when the DL SS is less than -160dB 🙂