Don't do it like that.
Instead, keep a class level DateTime value which you set to DateTime.Now when you start the timer, and generate the elapsed time on the Tick:
Dim diff As TimeSpan = DateTime.Now - startTime
Label1.Text = diff.ToString("hh\:mm\:ss\:fff")
Since Tick events happen at some point after (but not guaranteed to be exactly at) the moment when the timer expires, it's the only way to keep the elapsed time display accurate.
[edit]Code changed to VB[/edit]