When purchasing Premium Time, the time displayed will only show one unit of time (Day, Hour, or Minute).
Example
If you purchase 3 days of Premium Time and 12 hours have passed, your Garage will only show 2 days on the Store tab, despite having 2 days 11 hours and 59 minutes left.
Once your Premium Time reaches 12 Hours, then your Store tab display in your Garage will go from 1 Day to 12 Hours and continue counting down the hours from there.