They do a weird conversion in my experience. I think for the first “minute” it just counts all numbers in the seconds field as seconds, then as it counts down, the next minute is only 60 seconds.

Eg I think I’ve tried putting in 90 seconds, and that is actually 90 seconds, same as if I were to put in 1:30 (minute and a half)

So I might guess something like 4:90 == 5:30

The first seconds counter has to run out before the next minute counts down adding 60s to the seconds counter.

So 99:99 is the same as 1:40:39

Yeah my microwave lets me put 99 seconds in, and that's the highest I will go. In fact, I only put multiples of 11 in, because it's the quickest, just bing-bang-start.

On the rare occasion I'm microwaving something that requires, say, 5 minutes, it gets 444 and we hope for the best.

From my experience the numbers left of the colon are minutes and the numbers to the right are seconds. The seconds count down to 0 and then reset to 60 as the minute count decreases by 1 For whatever reason I actually get a tiny kick out of this weird behavior and almost always set my microwave for X minutes and 99 seconds (also almost never 100%power), my favorite part is adding 30 seconds with X:60< <70 so in theory it should be able to just go back to 9x but it always jumps up a minute and adds 30 instead. It knows 99 seconds is silly but still let's me do it.