The values in cells A1 & B1 are in days and fractions of days. If you put in 03/11/2009 12:00 (that's noon), Excel preserves that value as 39883.5
B1-A1 is the finish date/time - the start date/time, so if you start at 3/11/2009 12:05 and end at 3/11/2009 12:22 the math works out as
39883.5152777778-39883.5034722222 = 0.01180555555
Multiplying that value by 24*60 gives the number of minutes used (17)
Using the ceiling function with a 5 as the second parameter rounds the first parameter (17) up to the next multiple of 5 (so 17 minutes would be converted to 20)
dividing that by 5 to convert to units results in 4