How does the DateAdd function in ASP Classic work with the "ww" weeks option?

Question

I have to replicate functionality in C# from an old asp classic codebase.

The codebase seems to assume that the year can have up to 54 weeks and uses the DateAdd method with the "ww" option to figure out the "week ID" for purposes of categorising some files.

All of the online documentation I have found seems to ignore this question and a straightforward reading of it suggests that adding weeks is the same as adding (weeks * 7) days, however, I don't want to break backwards compatibility. I think that the old code has an off by one error but I don't want to assume it. It's possible for a year to have its days in 54 different weeks but I don't think that doing a DateAdd with a value greater than 52 ever makes sense.

My question is, how does the

DateAdd("ww", x, date) 

work?

Is it the equivalent of

date.AddDays(7 * x) 

in C#?

Are there edge cases where the first of January falls on (for example) a Saturday and DateAdd("ww", 53, date) could return a valid date in the same year?

Answer 1

I can tell you that DateAdd("ww", i, date) is equivalent to DateAdd("d", 7*i, date); therefore, provided that both functions in C# and vbScript have the same meaning, they must be equivalent.

To ensure DateAdd("ww", i, date) is equivalent to DateAdd("d", 7*i, date) I used the script bellow, it cycles 2 years and for each day, it cycles for 104 weeks if adding days or adding weeks mismatch somehow, it raises an error.

date0 = Date()

For i = 0 to 2 * 365
    date1 = DateAdd("d", i, date0)
    For j = 0 to 2 * 53
        date2  = DateAdd("d", j * 7, date1)
        date3  = DateAdd("WW", j, date1)
        assert = DateDiff("s", date2, date3)

        If assert <> 0 Then
            errMsg = _
                "OPS addDays And addWeeks unmatched" &_
                vbCrLf &_
                CStr(date1) & vbTab &_
                CStr(date2) & vbTab &_
                CStr(date3) & vbTab &_
                assert & vbTab

            Call Err.Raise(vbObjectError + 10, "Test DateAdd(""ww"", i, date)", errMsg)
        End If
    Next
Next

So I guess you can use date.AddDays(7 * x) to replace DateAdd("ww", x, date) with no concerns in mismatching dates.