You want to display the time between today and the start day and time? To start, set up a timestamp field that combines the date and time in the same field. You may want to set up a calculation field for this with the following calculation:
TimeStamp ( date ; starttime )
So you can continue to use separate fields to enter a start date and time.
However you choose to set up that timestamp, you can then set up an unstored calculation field like this:
YourTimeStamp - Get ( CurrentTimeStamp )
And the difference thus calculated is the number of seconds from now until the recorded time stamp.
This is just the first part of what you need to produce the display that you want, but all I have time to describe at this point in time.