When I tested it long ago, it took 2 seconds for the radiation to start hitting.

So a 2 second heating did nothing, a 3 second heating was actually 1 second, but a 60 second heating was 58 seconds which is close enough.

My wife has a strategy. If it needs 10 seconds, she hits 11, which lets her hit the same button twice instead of moving over to the zero and maybe missing. Same for 22, 33, 44 seconds. They're all close enough.

The way I tested it was to put a piece of wire in and watch for it to spark. I used a twist tie, bent in a rabbit ear antenna shape, at approximately the half wavelength. don't try this at home.