Is it possible to make the timer count backwards.

Not the blue sensing timer. But you can certainly create a timer that can count backwards by either subtracting the blue sensing timer from whatever you set your time variable to. Or you can set a time variable to a number and then change it by -1 or whatever you need.

Here is a simple timer that I created last week. See if this helps. The first one uses a wait 1 second block.

https://scratch.mit.edu/projects/207028077/

This one below uses the blue timer from sensing.
https://scratch.mit.edu/projects/208674889/

Make this script :

when I receive [game start]
set [time] to [100]
repeat until <(time)= [0]>
wait (1) secs
change [time] by (-1)
end