Countdown Timer

If you have created a simple countdown timer and you want to use it in Premiere Pro, by far the most efficient way to do that is to open the Essential Graphics workspace, select the timer layer, add any controls you may need in Premiere Pro to the Essential Graphics Panel, then save the comp as a MOGRT. It will render faster, be easier to use, and have a smaller file size than any other solution.

The second thing I would do would be to add the timer comp to the Render Queue and choose the Lossless With Alpha preset in the Output Module, then render a file that you would import into Premiere Pro and add above the shot or shots you want to have the timer run over.

The very last way I would approach the project would be to save the AEP file, import that into Premiere Pro using Dynamic Link, then pick the timer comp from the assets and add it to the timeline. Dynamic-link is the least reliable, slowest rendering, and most difficult solution to the problem. It works pretty well with simple things like a single-layer timer, but the more complex the FX shot is the more unreliable Premiere Pro becomes. 

I suggest saving the After Effects project with the timer.  Then importing the After Effects Project/Compostion into Premiere.