but I really can't find a timer function!
There is a TimerOne library.
You could avoid the millis() jumping issue by using micros(). millis() jumps periodically because the clock ticks 1024 times then millis increments by 1. After 41 to 42 iterations, the extra 24 ticks add up to a whole millisecond.
this is really weird
where is TimerOne?
is there a difference?
well, it's just that over 10 seconds it yields a 0.2 seconds error.