When trying to check for errors on a Customer Journey where the start date was set to the past, an error message pops up "The customer journey start date must have some slack."

I heard on one of the previews calls in answer to a question I asked that it was possible to put an earlier date/time in the Start Date field in order to force the Customer Journey to start immediately. Apparently this is not true according to this error message. If that is the case, then the error message should be more precise. How much "slack" time is needed? 1 minute? 1 hour? How do I force a Customer Journey to start immediately?