JeanRage's attachment has a good explanation, which I'll repeat and expand here.

I am using an array formula (search Excel help for "array formula"). The array formula is a powerful feature in Excel that allows a limited type of iteration for all cells in a given range. In this case, we need to know the minimum time. The formula to look at a date/time in B2 and return time is:

=$B$2-INT($B$2)

We need to do the same thing for multiple cells and then find the min. To do this, an array formula will create an array of values that is the result of this calculation, then we apply the function to the entire array. To start along this path we create the formula:

=MIN($B$2:B$4-INT($B$2:$B$4))

For Excel to interpret this as an array formula, you must enter the formula by pressing CTRL-SHIFT-ENTER. (If the formula already exists, you must press F2 to edit then press CTRL-SHIFT-ENTER.) Then you will see

{=MIN($B$2:B$4-INT($B$2:$B$4))}

The braces are added by Excel when you enter an array formula; you cannot type them in yourself.