Timer in Prospective Memory Experiment

Hello,

I'm working on an experiment for my Bachelor Thesis. I'm testing prospective memory and would like to add time-based prospective memory tasks. 

The test person is supposed to press the key "m" after 3, 5 and 7 minutes.

Also I'd like to add the possibility for the participant to see the time that has passed since the start of the experience pressing the key "t". Is that even possible?

Thanks in advance!

Have a great day.

Adele