Get day number within calendar week for a specific date - excel

Question

I have a data set which includes dates.
I need to split this out by week number for reporting purposes.
What I have so far is:
startDate variable containing 03/01/2015 (populated from data in spreadsheet)
startDay = Day(startDate)
startMonth = Month(startDate)
startYear = Year(startDate)
startWeek = Application.WorksheetFunction.WeekNum(DateSerial(startYear, startMonth, startDay))
which gives me week 1 in startWeek
However I know need to know how far into week 1 the date is.
So for this example, as the date is the 3rd of January, it includes 3 days of week 1
Meaning the reporting I'm putting together will only report on 3 days (as opposed to the full week)
The only way I've figured to do this so far is to calculate which day of the year the date is and the use a MOD calculation (basically divide by 7 and the remainder is how far into the week it is)
dayNumber = DateDiff("d", DateSerial(startYear, 1, 1), DateSerial(startYear, startMonth, startDay)) + 1
dayOfWeek = dayNumber Mod 7
This does work, but I was wondering if there was a nicer solution than this.

Answer 1

You could use a loop to determine how many days before startDate the week number changed:
Public Sub FindDaysInWeekNo()
Dim startDate As Date
startDate = DateSerial(2015, 1, 3)
Dim startWeek As Integer
startWeek = Application.WorksheetFunction.WeekNum(startDate)
Dim i As Integer
Do While startWeek = Application.WorksheetFunction.WeekNum(DateAdd("d", -i, startDate))
i = i + 1
Loop
Debug.Print i '= 3rd day in this week number
End Sub
The following table shows my comparison to the other suggested formulas and why I think that (refered to =WEEKNUM) my calculation is correct.
Note that if you assume 1st to 7th January will be week 1 (days 1 to 7) you cannot use the WeekNum function because this will give you a different result (see table above and note that the first week has only 6 days according to the WeekNum function). Also you cannot name this week number (as what everybody calls week number is defined as https://en.wikipedia.org/wiki/Week#Week_numbering).
Instead you will need to use …
Public Function AlternativeWeekNum(startDate As Date) As Integer
AlternativeWeekNum = WorksheetFunction.Days(startDate, DateSerial(Year(startDate), 1, 1)) \ 7 + 1 'note that this no normal division but a integer division and uses a backslash instead
End Function
to calculate the week number your alternative way, and …
Public Function AlternativeWeekNumDay(startDate As Date) As Integer
AlternativeWeekNumDay = WorksheetFunction.Days(startDate, DateSerial(Year(startDate), 1, 1)) Mod 7 + 1
End Function
to calculate the day in the alternative week.

Answer 2

You can use the Weekday() function for this:
=WEEKDAY(B4;2)
The second parameter mentions how you want your days to be counted (starting from Sunday or Monday, counting starting from 0 or from 1, ...).

Answer 3

dayOfWeek = (8 + Weekday(startDate) - Weekday(DateSerial(startYear, 1, 1))) mod 7
Just take the positive mod 7 of the difference between the current Day-Of-Week and the Day-Of-Week for the 1st January of whatever the year is