Subtracting two cells that have time and date

hey guys im really stuck.I have a bunch of data that has 2 columns, each with time and date information. I need to subtract one column from another, and need just the difference in time, in units of minutes. for example, i would need to subtract:

[2012-02-14 18:31:00.000] - [2012-02-14 07:26:00.000] = [785]

does anyone know how to do this? Thank in advance. I've attached a small sample of the two colums