Calculating how many minutes there are between two times

Try this

DateTime startTime = varValue
DateTime endTime = varTime

TimeSpan span = endTime.Subtract ( startTime );
Console.WriteLine( "Time Difference (minutes): " + span.TotalMinutes );

Edit:
If are you trying ‘span.Minutes’, this will return only the minutes of timespan [0~59], to return sum of all minutes from this interval, just use ‘span.TotalMinutes’.