I'lll time you (on)my watch这里为什么用on 而不是by?

I'lll time you (on)my watch这里为什么用on 而不是by?
I'lll time you (on)my watch这里为什么用on 而不是by?

I'lll time you (on)my watch这里为什么用on 而不是by?
我明白你的意思.
1 I'lll time you (on)my watch== I will record your time on my watch.
(把你所用的时间定格在我的手表上)
2 It is five by my watch=== My watch says it is five o'clock.
by=== according to