One way, using a macro. It will place the new values in the column to
the right of the originals:

Public Sub ConvertDates()
Dim rCell As Range
For Each rCell In Range("A1:A" & _
Range("A" & Rows.Count).End(xlUp).Row)
With rCell
If IsDate(.Value) Then
.Offset(0, 1).Value = Format(.Value, "yyyymmdd")
Else
.Offset(0, 1).Value = Right(.Text, 4)
End If
End With
Next rCell
End Sub

Note that this is based on the forms in your data.

If you have something like

01/30

meaning January of 1930, this macro will give an incorrect answer.

In article ,
"BernieH" wrote:

I have a *.CSV file with a column of dates in various formats, such as -

1/01/1900
1/07/1901
1/06/1903
1880
1901
1904
1911
1911
1913
1/05/2004
Winter 2004

I need to get these dates into YYYYMMDD or YYYY format (depending on how
much info is available, e.g. 1911 is OK left as 1911). Formatting the cells
as custom YYYYMMDD however doesn't work, as ?pre-1900 dates get converted
wrongly.

There are ~20K lines in the file, so any solution has to be pretty much
automatic.

Your help would be very much appreciated!

TIA

bernieh