I never use the timecode effect because it's too limiting. Look here: Dan Ebberts's Expressioneering Design Guide Universal Timer
Save this expression as an animation preset and use it often. Change the speed of the clock by changing the rate. Count up by removing the minus sign from the rate. set the start time, add or subtract decimal points from the milliseconds...
Just carefully read the article. If you must have frames let me know and I'll show you how to modify the expression for source text.
You apply the expression to the source text property. Just create a new empty text layer. Select the text layer in the timeline and press the U key twice to reveal the Source Text, then go to Animation > Add Expression and paste.
As an alternative you could add this Animation Preset by saving it to your custom presets folder, selecting the text layer, and then going to Animation > Apply Animation animation preset and browse to the preset. This will give you sliders and check boxes to control the universal timer. You can display hours, minutes, seconds, milliseconds or timecode, count up or down at any speed.
If you don't know how to use animation presets or apply expressions please do some research. It's pretty easy to use animation presets and to understand them if you spend 5 minutes with the help files.