How to tell the time?

I have an Excel 2007 workbook that I require to be able to €˜countdown the
time remaining until the scheduled start time of a market event.

I require that the function returns a value dependent upon the amount of
time remaining.

Time Cell Return
Start Time - 10+ Minutes A
Start Time - 5 to 10 Minutes B
Start Time - 2 to 5 Minutes C
Start Time - Under 2 Minutes D

I require assistance with both the formula to use and any relevant
information regarding the cell format / data type for cells B3 and B4 in the
example below

The time format I have used is HH:MM:SS

In the example below the time remaining is calculated as 7 minutes & 25
seconds, I need to know how to calculate this time difference and how to
convert that into seconds (445 Seconds) for use in the Vlookup function
(D1..E4) to return the letter B


A B C D E
1 Current Time: 13:52:35 600 A
2 Sheduled Start: 14:00:00 300 B
3 Time Remaining: 00:07:25 120 C
4 Return: B 0 D


Many thanks for your help