I have 6381 courses that have no start date in my Moodle installation. I have not noticed this, and it doesn't really seem to be a problem so far. In the database the start date is '0'.
When the course settings page is opened, the course start date always shows as the current time.
I am wondering how it would act on the dashboard when selecting 'past, in progress or future' courses.
But... is this a problem anywhere else? It might have happened by creating the courses using api/webservices and not adding the course start date field correctly.
Please let me know your thoughts on this