Did you hear about the Zone Rouge, which was created after WW1?
According to the Sécurité Civile, the French agency in charge of the land management of Zone Rouge, 300 to 700 more years at this current rate will be needed to clean the area completely.
If it doesn't matter if it's a few seconds too late or too soon an ATtiny 85 is good. If you need more accuracy you probably need an additional RTC.