I went on the App Store and just searched for contraction timer and downloaded the first free one I saw. Found it a complete pain in the ass as my phone locks itself after a minute so I kept having to constantly touch the screen to keep my phone on!
Is there a particular reason you want to time everything so accurately? Even midwives don't really time contractions, they mostly look for '3 in 10', etc. So they count how many in a window of time, rather than timing a contraction.
I only ask because, in some people, focusing on mathematical information like that can be counter productive to labour actually establishing. Equally for some people they can be utterly brilliant, but just worth bearing in mind if this is your first in case you find you might be falling into the first group on the day?