To open your timer in fusion for further modifications you'll have to select your title then you have to go over to Fusion by clicking the Fusion icon in your inspector. Once you are in Fusion in your inspector you can go to Modifier where you'll find the possibility to tick boxes to add/remove the hours/minute/second/frames.

>Select Title >Fusion >Modifier >Current Title.

And that's pretty much it, you can obviously do all the modification that you want directly in fusion but we're going to go back to the edit page if you're a beginner.

Now we're gonna create the style of our timer. I'm gonna start by selecting the font, then I'll make my title a little bigger, I'll add a bit of space between each number by using the Tracking slider. It give us a clean looking timer. 

So what if you want the timer to come down instead of counting up?

There's two possibilities, the simple way is simply to create a compound clip here we're gonna create a compound clip and then we basically just gonna reverse it. 

Then right-click on it and select retime control, in the pop up window click on reverse, what it does is basically reversing our clip and here counting down instead of up.

>Right Click >Reverse Segment.

The problem is that with this technic your title isn't responsive anymore, it will not adjust to your timeline.

This technic is the most simple but it would not be my preferred method, the best option is the following. 

We'll have to do it in fusion. We're adding another node, to open the window to search the nodes just hit (shift + space), then search for the node time speed.

>Go to Fusion >Press Shift + Space >Time Speed >Set the time. 

So, let's add that and here in time speed in the speed section you have to just put negative -1 instead of 1, and what it does is it just reverse basically the timing of our title. 

So, if we go back to the edit page we achieved the exact same thing it's counting down and at least now if we want to adjust it's responsive.