I recently worked on a control panel for a large commercial oven and asked the design team if they wanted the ability to enter "99" as a valid time (1m39s). They said their customers expected it.

They also said that converting it to 1:39 and counting from there was disorienting, so it actually had to time down while showing :99, :98... What an odd corner case. But your microwave does the same thing, go try it.