I got roped into being a time keeper for my clubs junior section. Tried my phone and found it worked but you can not see it in bright sunlight which is kind of a problem when you are track side.
I brought a Digitime 2000 from Up and Running and it is brilliant. Will store 2000 split times and you can save the data. Therefore, you can time several races one after the other and keep all the times.
It also has a port on it so you can connect it to a PC or printer and download the times you have saved. I