How to find out the number of Mondays in a month?

[sueb]

I need to count the number of each weekday in a month (# of Mondays, # of Tuesdays, etc.). Is this even possible in Access?

Let me see if I can explain the reason I need these 5 counts.

In our Operating Room, the time is allocated to different classes of surgeries (General, Neurology, Pediatrics, etc.), and we want to compare the actual minutes spent in each category with what was allocated, and the allocations are made per day-of-the-week.

Complicating the issue is that different amounts are allocated on different weekdays; for instance, General surgeries are allocated 450 minutes a day, except that it's allocated only 420 minutes on Mondays. Ob-Gyn is allocated 450 minutes on Tuesdays and Wednesdays, and Plastics is allocated 240 minutes on Tuesdays and Fridays, and 450 minutes on Thursdays. You get the idea.

So I can calculate the correct "monthly" allocation only if I can figure out how many of each weekday occurs in a given month.

Any advice will be appreciated, even including "can't be done in Access". However, if that's the answer, I'll be most grateful for tips about how to accomplish this "outside" in such a way that I can use the results within Access.

Thanks!

[Rabbit]

First you calculate how many days it takes to get to the next Monday. Then you subtract that from the amount of days in the month. If that's less than 29, there's 4 Mondays in the month, otherwise it's 5 Mondays. The same goes for the other days of the week.

[sueb]

OOh, I like that! But let me make sure I understand how this is working: Wouldn't that "29" have to change depending on how many days there are in the month? I looked for a VBA function that would give me that number, but couldn't find it. Is there one? Or do I have to set up a table that just contains the number of days in each month (also thinking about leap years)?

[NeoPa]

A routine to determine the number of particular days of the week in a given month might go something like this :

Public Function NumWeekdays(ByVal datThis As Date, _
                            Optional ByVal intDay As Integer = vbMonday)
    datThis = DateAdd("d", 1 - Day(datThis), datThis)
    intDay = (intDay Mod 7) + 1
    intDay = Weekday(datThis, intDay) - 1
    NumWeekdays = DateDiff("d", datThis, DateAdd("m", 1, datThis))
    NumWeekdays = (NumWeekdays + intDay) \ 7
End Function

It's not too easy to explain as it gets a bit fiddly, but if you look through it you might pick it up.

[sueb]

@Rabbit, so what I'm going to do, I think, is calculate the number of days in the month by subtracting 1 day from the first day of the next month and extracting the 'day' from that calculated date.

However, it turns out that I don't know how to construct a date. I want to have a code something like:

Public Function Count_Weekdays(Month_Start As Date)
On Error GoTo Err_Count_Weekdays

    Dim Next_Month
    Dim Next_Month_s_Year
    Dim Days_This_Month As Integer
    Dim Days_To_Next As Integer
    Dim Monday_Count As Integer

    ' Get number of days in the month
    Next_Month_s_Year = DatePart(yyyy, Month_Start)
    If DatePart(m, Month_Start) = 12 Then
        Next_Month_s_Year = Next_Month_s_Year + 1
    End If
    'Next_Month = (1, datepart(m,Month_Start)+1, Next_Month_s_Year)
    'Days_This_Month = day(Next_Month-1)
    
    ' For each weekday:
    '   Calculate number of Days_To_Next Monday
    '   if Days_This_Month - Days_To_Next < Days_This_Month - 1 then
    '       Monday_Count = 4
    
    ' Calculate minutes allocation for each Surgery type for this month
    '(make an update query to update a table holding these values?)

but I don't really know how to write line 15. (and I'll worry about what follows after I get at least this part right!)

[sueb]

Oh, NeoPa, I might have known you'd just have this in your back pocket!

[gershwyn]

This is a function I use all the time that will calculate the number of days in a given month:

Public Function DaysInMonth(Month, Year)
  DaysInMonth = Day(DateSerial(Year, Month + 1, 0))
End Function

The dateSerial part is a little tricky, but I'll explain. The last day of the current month is equivalent to the day before the first day of next month. DateSerial(Year , Month+1, 0) will give us that day, and the day function extracts just the day portion (you could also use DatePart for the same thing, but I think this is cleaner.)

You could do something similar, using the Weekday (or DatePart) functions to figure out what day of week the first day of the month was, and that should give you the other piece you need to calculate the number of days using Rabbit's method.

Edit: I see the Bytes ninjas are in full force today. You anticipated my function before I posted it and NeoPa has a full answer already.

[sueb]

Wow, Gershwyn, this little function is a real eye-opener for me--I didn't know the DateSerial function at all. Does this get the right year when Month is December? (I mean I assume it does, or you wouldn't use it "all the time"...)

[gershwyn]

Yes, it is smart enough to wrap around to the next year if needed. So if you specify a month of 12, it will calculate (Month + 1) = 13 and proceed with January, the same way it knows to treat a day of 0 as the previous day. It also handles leap years wonderfully, without having to code for the exceptions.

Test case:

?DaysInMonth(1, 11)
 31 

?DaysInMonth(2, 11)
 28 

?DaysInMonth(12, 11)
 31 

?DaysInMonth(2, 12)
 29

[NeoPa]

It certainly does Sue. See the simple test below (run in the Immediate pane of the debugger).

?CDate(DateSerial(2011,13,0))
31/12/2011

[sueb]

@Gershwyn: that's lovely--very elegant. Okay, I'll try out these options and post back here which one I end up using.

[sueb]

Okay, I'm going with NeoPa's code for now (although I saved Gershwyn's off because I think I'll need it for another project later).

I now have more questions about how to accomplish my final goal of comparing "actual" with "allocated" minutes, but I'll ask those in other threads.

Thanks, everyone!