After effects Timeline display: frames into milliseconds
Here's a workaround:

Create a left aligned text layer - type a zero. Format to the size and colour of your choice.
Twirl down this layer, twirl down text, ALT click the stopwatch on the Source text property. In the expression field that opens in the timeline, replace the text with:

Math.round(time*1000)+" ms"

click out of the text field into a blank area of the timeline.

The text will now show the time of the current frame in milliseconds.