DATEVALUE function

[Welsh Mark3]

I have a column of data which I thought were dates but the data doesn't seem to behave as such.
I have attempted to use Datavalue function but no am getting #value !

I believe I have checked my system date and time settings, Doesn't anyone have any advice?

 

[JoeMo]

Can you post some examples of your "dates" so we can see the format. If you have a "date" in cell A1, what does =ISNUMBER(A1) return when entered in any empty cell?