Even though the commit has a proper date (correctly displayed in "Commits" or "Branches" view), the Pipelines status bar still shows is as "Invalid date by <commit author>".
I checked a number of my repositories which have Pipelines enabled and identified a pattern: whenever there is a manual step(s) still to be triggered, it is "Invalid date...", however if there are no manual steps in the configuration or all manual steps have been executed already, it shows correct message.
Also, in my case all manual steps are used for deployments (i.e. have
deployment: <stage name> in the yaml config)