C# - Duration between two DateTimes in minutes

I need to determine duration between two DateTimes in minutes.

However, there is a slight twist:

• exclude weekends
• only count minutes which are between 7:00AM and 7:00PM. for example: [09/30/2010 6:39:00 PM] - [09/30/2010 7:39:00 PM] = 21 Minutes

I'm just having a hard time coming up with a decent way to do it and would appreciate if anyone can suggest something.

Thanks.

Edit:

I ended up going with dtb's solution. There is only one special case which needed to be taken care of: if end time is after 7:00PM, count the minutes from 7:00AM to the actual end time.

This is how I modified it:

var minutes = from day in start.DaysInRangeUntil(end)
                where !day.IsWeekendDay()
                let st = Helpers.Max(day.AddHours(7), start)
                let en = (day.DayOfYear == end.DayOfYear ? 
                            end :
                            Helpers.Min(day.AddHours(19), end)
                            )
                select (en - st).TotalMinutes;

Again, thanks for the help.