Do...while Loop: Check Until So Many Cells Have Been Marked Red

i'm having a bit of a issue with a do while loop. It might just be my complete lack of understanding, but i'll briefly explain what I am doing before I show you the code. I am marking cells with what I consider an invalid type for that cell red. Well unfortunatley in some sheets the number of invalid types is so high the macro crashes. So my bright idea was to inclose the check statment into a do while loop, that will only check until so many cells have been marked red. So I created a public variable, assigned it a value of 1. I then assigned the macro that marks the cell red to also take y and add 1. So in theory (mind you what I take for theroy might be in my head) it should only run until the paramaters of the do while are met which in this case are do while y < 20.

y = 1
Do While y < 20
Set MyRange = Intersect(Columns(7), ActiveSheet.UsedRange)
For Each r In MyRange
CheckDate
Next r
Loop

That is the snippet from the program calling the function, and here is the function

Public Function CheckDate()
If IsDate(r.Cells) = False And IsEmpty(r.Cells) = False Then
r.Select
MarkDate
y = y + 1
End If
If r.Cells < 1 / 1 / 1910 And IsEmpty(r.Cells) = False Then
r.Select
MarkDate
y = y + 1
End If
End Function

what i'm doing wrong here? Oh and y is assinged as a public varaible, integer data type.