There might be other ways to do this, but here's one:
First off, make sure you are using the Time Date generator as opposed to the Timecode generator.
The animation check box is in the Inspector Window. If you uncheck it you can then set a keyframe with the value you want to start with and then move to the point you want it to end and add another, specifying the end value.
Since Motion's generator doesn't seem to let you not display the hours, you'll probably want to either crop the layer or use a mask. Keep in mind that, depending on the font you use, the time might jump around a bit. Also, it doesn't really seem to matter at what point in time you place the ending value, Motion will compensate to get it there even if it doesn't truly represent the time elapsed. In other words, make sure your ending point is in the right spot if the timing is critical.