Counters used as timers are very accurate for long time durations. This is because the input signal opens a gate to let 1µ Sec pulses through which the counter counts, so if the signal is 1 pulse per second, it can count up to 1,000,000 with an accuracy of 1 part in 10^8 or better. If the input signal is at a 1KHZ rate, the counter can only count up to 1,000, so its accuracy is 1 part in 10 ^3.
As counter use the longest gate period possible, i.e. 10 Seconds, this way the indicated frequency is +- .1 HZ, if you use 100µS, the accuracy will be +- 10 KHZ
Frank