Scratch

[When green flag clicked]
[set [Timer] to (0)]
[forever]
[][wait (1) secs]
[][change [Timer] by (1)]

You can change it to [wait (0.1) secs] and [change [Timer] by (0.1)], but you choose. It depends on what accuracy you want...

You can try using the timer blocks that come with Scratch (look in Sensing).