Thanks paul. I am planning to build a temperature datalogger, that will collect data every two minutes for a month or two, so I need to get it as accurate as possible. Also, in my country, Argentina, there are not RTC availables....

Now following Ronsimpson way, (the second one, with the 20.000 MHz), the variable to which we add 859 on every interrupt, I should never clear it, right??? Or should I clear it every time the variable overflows???

Sorry for my english, and thanks again.
Manuel