Perhaps I am not following your idea clearly, but it seems the problem is bigger than you state...

Up until 599 seconds the display may read minutes and seconds like: M.SS with no problem, but with 10 minutes, then seconds (or minutes) must be only one digit. You may choose to change the display so that at low times, - less the 599 seconds you display M.SS but at 600 seconds the display becomes MM.S. This will require two display routines. Or, you may display minutes as hexadecimal and use A, B, C, D, E, F for values greater than 10. Then you may display M.SS for all values up to 999 seconds with only 1 minutes digit.

Once you have determined how to display your time consistently, then recheck the datasheet of your display, it seems strange to me that a serial display will require more than some string data to display numbers. I would imagine some SEROUT Pin, Mode, (Item{,Item}) statement would work - at least try this through the built in serial function (DEBUG) in PicBasic to be sure all that you intend to send to the display is correct.

HTH