Diagnosis
You have attempted to change a tax credit record which lies within the
previous tax year. This change could cause changes to payments already
reported on statutory reports (e.g. P60 and P35).

System Response
If the end date or the certificate of payment date is in the previous
tax year, this message is displayed as an error, because this action
will affect payments already reported on. If only the start date is in
the previous tax year, this message is displayed as a warning, because
this action may affect payments already reported on.