Use this format
hh:nn:ss.000

Access won't be fast enough to show every millisecond.  Even if you set a form's timer interval to 1 ms (the minimum), Access will take more than 1ms to run the event, thus unable to count every ms.  You need to do this kind of thing in a different, more capable coding environment, such as javascript.  Google "clock with ms" to see websites that can show this.  And the code is quite simple too.  The method is the same: set timer interval to 1ms and show the time.  Except that javascript is fast enough to do it, and Access isn't.

This from Microsoft:
The resolution of the GetTickCount64 function is limited to the resolution of the system timer, which is typically in the range of 10 milliseconds to 16 milliseconds. The resolution of the GetTickCount64 function is not affected by adjustments made by the GetSystemTimeAdjustment function.

If you need a higher resolution timer, use a multimedia timer or a high-resolution timer.

Seach for "GetTickCount64 function" in Google and the web page has links to the more resolution solutions.

Might make an interesting video. Hmmm....

@ Kevin, Because you said please, there's the example above.
If you want the code for the two procedures (fTimer & fDispElapse) then you'll have to ask with a "pretty please" and Richard will have to add me to the Access Developer Network>Other Access Consultants page.

The fTimer function has ten timers and up to ten laps per timer. The procedures are a little complicated, but well documented.