I would suggest that you split the tasks into three part:
- displaying from a buffer: run this part from a timer interrupt so it is done transparently. All you need is to stuff font information into the buffer;
- time keeping: put this in another timer interrupt to keep track of count-down time.
- processing: do this in the main loop. Process buttons, process time keeping and convert it to the right font info into the display buffer. For example, if the time is less than 10 minutes, put 'blank' into the 1st char of the buffer, '8' into the 2nd char of the buffer, '3' into the 3rd and '2' into the fourth to display ' 8:32'. To pause, you just need to pause the time keeping function.