We had a microwave in the office that auto started based on whatever number you pressed. If you wanted to type in 45 seconds, you would press 4 and it would instantly start running with 4 minutes on the timer. Everyone just pressed the "+30 seconds button" until it was close to the time they wanted and then stopped it manually to get it more precise.
If I remember right the solution was to press "Cook Power" then type in the number of seconds. There was nothing on the microwave to explain this, I had to look it up online.
Our microwaves rotary encoder behind the knob is messed up and as opposed to fixing it (you have to turn it REAL slow or it'll jump really fast forwards in time or backwards all over the place) we have just used the +30 seconds for 4 ish years.
I mean, almost certainly you press "cook time", not "cook power" to set the time, but other than that this is exactly how every single non-dial microwave I have ever used works.
If I remember right the solution was to press "Cook Power" then type in the number of seconds. There was nothing on the microwave to explain this, I had to look it up online.