The following KB provides a possible reason why a user/admin may see an error indicating 'Planned end date must be after planned start date.' When submitting a change request.
The message is actually from a java class so the messaging is not something that would find directly from a script.
Release or Environment
The most common cause is that there has been a customization to the client script 'Planned Start/End Date Validation' where the logging is found. This is a client sided error that appears when the user submits the change request form.
The simplest resolution is to view the update versions of the client script and then revert back to the OOB version.