That first date, Is it consistently "formatted" as "nn mmm yyyy"?

Yes it is. The cell  is either blank, or, contains a date  of the format
dd mmm yyyy in text format.

Yes, a single VBA function could certainly return the format  you want
without any intervening cells. Steps you'd need to take:

Pass the original date  as a parameter to the function.

Use the Split function to split it apart at the blanks into a three-element
array.

Use a Case statement to convert Jul to the STRING "07", etc. Do not convert
it to the number 7 or you'll just need to put a leading zero back on it.

Use the Join function to join the bits back up, specifying a hyphen as your
separator.

Return this new string from the function (and therefore into the cell).

This should get you going I think. It alters a selection to YYYY-MM-DD.

Sub subYMD()
' Alter a selection to YYYY-MM-DD.

Dim rlSelection As Range
Dim rlCell As Range
Dim slOldDate As String
Dim slNewDate As String
Dim slAllMonths As String
Dim ilMonth As Integer
Dim slNewDateArray() As String

slAllMonths = "JANUARY" & "FEBRUARY" & "MARCH" & "APRIL" _
& "MAY" & "JUNE" & "JULY" & "AUGUST" _
& "SEPTEMBER" & "OCTOBER" & "NOVEMBER" & "DECEMBER"

Set rlSelection = Selection

For Each rlCell In rlSelection

slOldDate = rlCell.Value
slNewDateArray = Split(slOldDate)
ilMonth = InStr(slAllMonths, UCase(slNewDateArray(1)))
Select Case ilMonth
Case 1
slNewDateArray(1) = "01"
Case 8
slNewDateArray(1) = "02"
Case 16
slNewDateArray(1) = "03"
Case 21
slNewDateArray(1) = "04"
Case 26
slNewDateArray(1) = "05"
Case 29
slNewDateArray(1) = "06"
Case 33
slNewDateArray(1) = "07"
Case 37
slNewDateArray(1) = "08"
Case 43
slNewDateArray(1) = "09"
Case 52
slNewDateArray(1) = "10"
Case 59
slNewDateArray(1) = "11"
Case 67
slNewDateArray(1) = "12"
End Select
slNewDate = slNewDateArray(2) _
& "-" & slNewDateArray(1) _
& "-" & Format(slNewDateArray(0), "0#")
rlCell.Value = slNewDate

Next rlCell

End Sub

It works on
the currently selected range:

Sub blah()
For Each cll In Selection.Cells
a = Application.WorksheetFunction.Trim(cll.Value)
DateBits = Split(a, " ")
If UBound(DateBits) = 2 Then 'only a very basic checking that there
are two spaces in the trimmed date  string
DateBits(0) = Format(DateBits(0), "0#") ' comment out to return single
digit day of month
DateBits(1) = (InStr("JANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDEC", _
UCase(DateBits(1))) + 2) / 3 'month in year as number
DateBits(1) = Format(DateBits(1), "0#") 'comment out to return single
digit month
cll.Value = DateBits(2) & "-" & DateBits(1) & "-" & DateBits(0)
End If
Next cll
End Sub



There'll be some line wrapping above so unwrap them as required.
See also comments in the code.

Yeah... I thought about checking as well but then my thinking was that
Robert would only select the cells he wanted to alter.

I did think though that some of the months may be written out in full so I
used the full names for the Instr. My experience is that people sometimes
write Jul and sometimes July... Same for Jun / June ... Depending on the
price of eggs or wether it's Monday.

The split takes care of the Trim I think.

I thought it was a nice little problem.

"The split takes care of the Trim I think."
So did I, but on testing (XL2003) it didn't!

Call it brain-fade (or rushing out the door to go and pick up my car from
the mechanic). Yes, you can't use join unless you put the stuff into an
array in the correct order - and that's clearly more trouble than it's
worth. Sorry, 'bout that.