measuring response time between times

I'm brand new to ML and trying to measure people's ability to estimate blocks of time (e.g. 10 seconds, 5 seconds, etc.).

Ideally, I want to have a "start" and "stop" button on the screen and measure the interval between those responses. But, I've only been able to make it work by creating two items to do this (e.g. item 10 is start and item 11 is stop).

Any suggestions?

You could create a custom item to do this with HTML forms and some java. Or you could, as you said, just time the difference between two sequential items. Why is the latter a problem?