Not tried this before, but I remember reading on a similar forum to this once... I'm sure there's an easier way. Basically, the guy suggested creating separate image files in photoshop or whatever. You would have to create the separate numbers (i.e. minutes, seconds etc.) as separate layers. Then export one image for each second (e.g. 01, 02, 03...58, 59). And one for each minute that you'll be going up to. You will then need to create an extra couple of video layers in Premiere. You can leave each of the 'minute' images stretched out over a minute, however, the second images you will need to change for every second (sounds very tedious) it's probably easiest to do this by setting the default still image duration to 1 second. You'll have to make sure that you have transparency set on all of the images (use paste attributes to do this).

Sounds horribly time consuming, but I'm sure it would work well. I guess the important thing is to make sure the numbers all line up when you create them in photoshop.