A for/next loop produces a negative number

Hi,

I'm using the dates as markers: Every date marks a new batch of data for that date; the data is below the date until the point a new date is found. In this example the marked cells are dates (rows 1, 21, 31) with data from each date listed below the date. Everything below 8/24 is from 8/24; everything from 9/2 is from 9/2 etc. Step one is defining these date-markers.

Screenshot 2021-06-01 105654.jpg

Ok, so here's my macro :

Sub LoopTest1()
Dim TNV As Integer
Dim R As Integer
Dim NR As Integer
'
NR = Cells(Rows.Count, 1).End(xlUp).Row
TNV = 0
'
For R = 1 To NR
    TNV = TNV + IsDate(Range("a" & R))
    Cells(1, 4) = R
    Cells(1, 5) = TNV
    Next R
'
End Sub

NR = find the last row of data
R = for/next loop counter, which doubles as a row counter
TNV = 'the new value' that is added to each time a date is found. Since IsDate is a TRUE/FALSE or 1/0 result, I simply added each IsDate result to the TNV running total.

So I have it looking through column A while displaying the R and TNV values as the loop is running. The thing is TNV goes negative. TNV displays the correct number of dates (1, 3, 15 whatever), but in negative (-1, -3, -15 whatever).

I'm stumped.

Thank you.

cause all 1s in bits = -1 ... and not-0 stuff ... got it.
Thank you