Calculate DiffDays function DateTime order - Does the larger DateTime come first or second

Question

I am coding a MVC 5 internet application, and I am wanting to write a function that will work with both a SQL statement as well as with a List.

Here is my code:

public static class TestableDbFunctions
{
    [System.Data.Entity.DbFunction("Edm", "DiffDays")]
    public static double? DiffDays(DateTime? dateValue1, DateTime? dateValue2)
    {
        if (!dateValue1.HasValue || !dateValue2.HasValue)
            return null;

        return (double)((dateValue2.Value - dateValue1.Value).TotalDays);
    }
}

As you can see, the code calculates the difference in days between two DateTimes.

My question is this:

Should the code be

(dateValue2.Value - dateValue1.Value).TotalDays 

or

(dateValue1.Value - dateValue2.Value).TotalDays?

Thanks in advance.

Answer 1

Since you are trying to compliant with SQL, the closest analogue is DateDiff. The 1st argument is the start date and the 2nd i end date. The code should be

return (endDate - startDate).TotalDays;