Why does my microwave go from 100 to 59?

It is changing from 1 minute and zero seconds to 59 seconds.

It is counting SECONDS, like 1:00

It's not 100. It's 1:00

Layton, the 1 is one minute. That's 60 seconds.

This is the type of thought borne out of extreme loneliness and hungryman microwave meals. "Singles & Dating" amirite or amirite?

It doesn't.

It goes from 1:00 (one minute - otherwise known as sixty seconds) to 0:59 (fifty-nine seconds).