TIMR0 can use an independant external oscillator I think.
I haven't done it yet, but take a look at the datasheet. I saw something about it in the registers.

I was thinking of using 32.768 Khz watch crystal to be able to keep accurate time.

I might try it on a breadboard today if I can find the time and will let you know how it comes out.

There's also the possibility of using a RTC with 1/100th of seconds.