Open MsgBox if a date on birthday list matches today - excel

Question

Every time I try and execute my code it says "object required."
I want a MsgBox to open if one a date on my birthday list matches today.
The birthday list extends from b2 to b100 and I want to look through to find the birthday.
This is a piece of code I took from a YouTube video.
The MsgBox pops up every time I open the workbook.
Private Sub Workbook_Open()
Dim cl As Range
Set cl = ThisWorbook.Sheets("Birthdays").Range("B2:B100")
If IsDate(cl) Then
If Now >= cl Then
MsgBox "Somebody's had a birthday!"
End If
End If
End Sub

Answer 1

You are getting Object Required error because of a typo ThisWorbook should be ThisWorkbook
It is very normal to encounter such errors. So always use Option Explicit. I have covered it in To ‘Err’ is Human
So Can I have it added to my code automatically?
Yes you can. To have it added to all new files you create, simply select "Tools" -> "Options" in the VBE, and tick the "Require Variable Declaration" box.
Note: This will effect only new files that you create. You will need to add it yourself to existing files.
I basically just want my excel to create a msgbox when I open it, if one of the date on my birthday list matches today.
You can use Application.WorksheetFunction.CountIf to check if there is today's date in a range.
Sub Sample()
Dim ws As Worksheet
Dim rng As Range
Dim matchFound As Long
Set ws = ThisWorkbook.Sheets("Birthdays")
Set rng = ws.Range("B2:B100")
matchFound = Application.WorksheetFunction.CountIf(rng, Date)
If matchFound > 0 Then
MsgBox "Birthday Found"
Else
MsgBox "Birthday Not Found"
End If
End Sub
Screenshot

Answer 2

cl is a Range object representing 99 dalmatians cells, each encapsulating a Variant value.
The IsDate function is happy to take a Variant, but doesn't know what to do with 99 of them.
Because Range has a hidden default property, you can use it as if it were a value - but, especially for someone that's just beginning to learn VBA, it makes for confusing, implicit, "magic" code that says one thing, and does another.
If IsDate(cl.Value) Then
The implicit Range.Value member call here, yields a Variant representing the cell's value itself if the range represents only a single cell, otherwise (i.e. if the range is for more than one cell) it yields a Variant pointing to a 2D Variant array (in this case 99x1) that's holding every single value.
IsDate wants one value, so if we have 99 of them, we need a loop. But here's the thing: the last thing we want to do is iterate individual cells, get their Value, and verify that - because that would be very slow.
So instead, we grab that 2D Variant array, and iterate that.
If cl.Count = 1 Then
'single-cell range: not a 2D array
If Now >= cl.Value Then
MsgBox "Somebody's had a birthday on " & Format(cl.Value, "yyyy-mm-dd")
End If
Exit Sub
End If
Dim values As Variant
values = cl.Value
Dim currentRow As Long
For currentRow = LBound(values, 1) To UBound(values, 1)
Dim currentCol As Long
For currentCol = LBound(values, 2) To UBound(values, 2)
Dim currentValue As Variant
currentValue = values(currentRow, currentCol)
If IsDate(currentValue) Then
If Now >= currentValue Then
MsgBox "Somebody's had a birthday on " & Format(currentValue, "yyyy-mm-dd")
Exit Sub
End If
End If
Next
Next
Right now the msgbox just pops up every time I open the excel whether a birthday matches or not.
Sounds like your actual code has On Error Resume Next somewhere - that makes VBA ignore any run-time errors and merrily keep running, ...and you definitely don't want that. Rule of thumb, never use On Error Resume Next to side-step an error. Execution is normally halted when there's an "object required" error: an unconditional MsgBox popping means execution is allowed to continue in an error state, and that can't be a good thing.
As Sid found out, the type mismatch is caused by a typo -- this shouldn't be allowed to happen: make sure every module you ever type any code in says Option Explicit at the top, and it'll never happen again... for your early-bound code (late-bound code is still vulnerable to typos, but that's another story for another time).
Lastly, note that several of the above mentioned issues would have been reported by Rubberduck's code inspections (disclaimer: I manage this open-source project) - including the typo, the implicit default member calls, and the absence of Option Explicit.