The following VBA was the solution that I found.
Dim c As Variant
Set rangetoalter = Selection
For Each c In rangetoalter
'0.03653 chosen as this is slightly more than a leap year and ensures
'that date will be in the correct year.
If IsNumeric(c.Value) And Right(c.Value, 1) = 0 And c.Value > 19000000 Then
c.Value = (c.Value - 19000000) * 0.03653
'the macro will crash if on a date formatted square. Also some columns have pathology results in thus the number has to be greater than 1900. The error has occured if the right digit is a 0
i am sure this is not very elegant but it works