Do you know a library to do this?
it is generally very difficult to find a library that does precisely what your application wants. in fact, the purpose of a library is so that it is generic enough for you to tailor it for your specific application.
in this particular case, what you wanted to do is quite simple:
1) read through the datasheet;
2) figure out how the timer works;
3) figure out how the power management works;
4) configure timer isr to wake up the mcu periodically; an overflow isr is typically all you need;
5) run the task directly in the isr if is it short or set a flag for its execution in the loop;
6) upon completion, put the mcu to sleep.
all you need is to tackle the little steps above and you will get it done in no time, and learn a ton about your device.
device datasheet is the best friend of any embedded engineer, not the worst enemy.