Adding Timer to My Videos

Another possibility might be to take a video of a clock or timer and use it for the "picture in picture" feature.   

https://helpx.adobe.com/premiere-elements/using/creating-picture-in-picture-overlay.html

Hi,

Is a timer a countdown timer?
In that case

(The figure below is for the Japanese version.)

1.　Click the panel menu of the "Project Assets" panel (red frame below), and select "New Item" → "Countdown Mark" from the menu that opens.

2.　In the "Countdown mark setting" window that opens, set the color and beep sound for each part, and click "OK" (figure below).

3.　This adds a countdown mark asset to the "Project Assets" panel, and places a countdown mark clip in the "Timeline" panel with a duration of 11 seconds starting from the current-time indicator on video 1 track.