Type mismatch in vba Excel

Question

If I run this code I get an error but can't find the reason, is someone able to help me out please

If Format(DateAdd("d", (i - Weekday((CB_Mth.Value) & "/1/" & (CB_Yr.Value))), _
((CB_Mth.Value) & "1/1" & (CB_Yr.Value))), "mmm") = ((CB_Mth.Value)) Then
    If Controls("D" & (i)).BackColor <> &H99FF33 Then Controls("D" & (i)).BackColor = &H99FF33
    Controls("D" & (i)).Font.Bold = True

If Format(DateAdd("d", (i - Weekday((CB_Mth.Value) & "/1/" & (CB_Yr.Value))), _
((CB_Mth.Value) & "1/1" & (CB_Yr.Value))), "d/m/yyyy") = Format(ThisDay, "d/m/yy") Then Controls("D" & (i)).SetFocus
Else
    If Controls("D" & (i)).BackColor <> &H99FF33 Then Controls("D" & (i)).BackColor = &H99FF33
    Controls("D" & (i)).Font.Bold = False
End If
Next

End Sub

Answer 1

The error you are receiving is most likely due to your DateAdd functions expecting a date, However you are providing it with CB_Mth.Value & "1/1" & CB_Yr.Value which would come out as: <Month>1/1<Year>. I assume you ment <Month>/1/<Year>

Personal opinion: use ISO-8601 date format

Below is my attempt at cleaning up the code to read it. I may or may not have grouped your if statements correctly (not sure exactly what you were trying to do).

        If Format(DateAdd("d", i - Weekday(CB_Mth.Value & "/1/" & CB_Yr.Value), CB_Mth.Value & "/1/" & CB_Yr.Value), "mmm") = CB_Mth.Value Then
            If Controls("D" & i).BackColor <> &H99FF33 Then
                Controls("D" & i).BackColor = &H99FF33
                Controls("D" & i).Font.Bold = True
            End If
        End If
        
        If Format(DateAdd("d", i - Weekday(CB_Mth.Value & "/1/" & CB_Yr.Value), CB_Mth.Value & "/1/" & CB_Yr.Value), "d/m/yyyy") = Format(ThisDay, "d/m/yy") Then
            Controls("D" & i).SetFocus
        Else
            If Controls("D" & i).BackColor <> &H99FF33 Then
                Controls("D" & i).BackColor = &H99FF33
                Controls("D" & i).Font.Bold = False
            End If
        End If
    Next

End Sub