Patrick's suggestion about T2 was the best though.
Or use a derivative with a PCA or a CGA, with a PCA/CGA clocked throuh T2 you can actually get interrupts once a second.

Beyond that, if you're doing long time intervals, remember that the xtal isn't extact...either initially or with time and temperature.
For absolute precision use a TCXO (temperature compensated xtal oscillator abt $3 n big bunches if you use a cellphone frequency)

Erik