luckely i hadn't implemented it yet...downloaded 5553
After sifting through 2 years of data I can confirm that OS time is most accurate.
I'm testing 0.4.3 now that has the capability to set the RTC before reboot.
Although it doesn't make the clock 100% accurate.
For long stand-alone runs I think it has it's advantages to be able to correct the clock close to real world time with every reboot.