Countdown Timer Reset

Hello, I am new to Construct 2. I have found it fairly easy to pick up but I am stuck on creating a count down timer for a turret defense game. I am trying to get it to count down from 20 then once it reaches 0, it triggers the next wave then resets to 20. I can get the timer to count down and I can get the wave to trigger but the problem I have is in the resetting of the time. It is held in a global variable as 20. I do not want to reset the global variables nor do I want to restart the layout. I just need to have the time go back to 20 and start counting again. I have searched through the forums for days and have not found a satisfactory answer for this. Can someone help me with this or at least point me in the right direction, thank you in advance.

In the event where you trigger the wave, you are going to add another action. Go to System, and then Set Value. For the variable, choose your timer variable, and then for the value, you would enter 20. If you have your timer set up so that it is constantly counting down, this should be all you need to do.

Based on the information you provided, I think this should work, but if it doesn't, let me know more about how you set up your events.