I have an Action Items list that captures [Created], [Status], and [Date Completed] columns. [Date Completed] is an optional column to be entered at a later date when the Action Item is closed. If [status]="Completed", [Date Completed] must be greater than or equal to [Created]. If [status]=blank, no validation is needed and [Date Completed] can be left blank. How do I code this formula? I have tried to adapt several similar formulas posted on this site without success. In all cases, SharePoint displays an error that [Date Completed] must be > Created no matter what value is selected in [Status].
Use list validation for this. Make the column [Date Completed] a non required field. Then go to
List Settings -> Validation settings
=OR(AND([Status]="Completed", [Date Completed]>=[Created]), [Status]="None")
Make your message something about how Date completed needs to be filled in if the item is completed.