Here is a simple little method to calculate the number of minutes between two datetimes. Make sure to use TotaMinutes not Minutes in your return value.
public static double GetMinuetsSpan(DateTime beginDate, DateTime endDate)
{
TimeSpan DeltaMinutes = TimeSpan.Zero;
TimeSpan BeginDate = new TimeSpan(beginDate.Ticks);
TimeSpan EndDate = new TimeSpan(endDate.Ticks);
DeltaMinutes = BeginDate - EndDate;
return DeltaMinutes.TotalMinutes;
}