This is what I used. And it works. The problem is the timer using minutes and seconds and 1 second delay is a tiny bit faster than the countdown using centiseconds and delay 0.01
So it reaches the 10 seconds mark a little faster and obviously keep increasing the difference as time goes on.
Delay/timer execution can have small fluctuations in the execution time depending on framerate and frequency of the timer. There are also rounding errors when dealing with small increments that may affect the accumuated time.
I suggest you try the FromSeconds function and get the time from a single float: