struct timehands: th_generation field

[deco33000 at yandex.com]

Hi,

Currently I read more about timers and clock.

I am struggling on one field of a struct : http://fxr.watson.org/fxr/source/kern/kern_tc.c?v=FREEBSD10#L63

I would like to understand what th_generation means please.

Thanks a lot

-- 
Jog