You create a 0.1s time unit generator using Timer 0 and then RTC using software counter (counting seconds, ev. hours/days/months if that's your idea of storing time instead of "seconds from epoch") basing on pulses/interrupts generated by Timer0. (or use a separate self-powered external RTC chip which does all the work for you and frees your timers, plus keeps time when external power is off). You can stop/reset Timer 0 with an external serial signal if serial is up and working to receive it. (but if you use it just to generate 1s, you could just keep the 1s accuracy of the device and reset the software counter incremented by Timer0 overflow, without even touching Timer0 proper)


What exactly do you want to measure with Timer2?
(the capture/reload feature allows you to measure periods of time without stopping the clock.