I cannot think that the clovis date was considered to be the first day a person stepped foot in North America. I was under the impression that fossil find was indicative only. Meaning that if tools were fond to have been created on a certain date some time in the past therefore people had to be in the area some time before that date.

since man was killing Mastodons in North America 13,800 years ago we cannot therefore assume that they arrived in the continent 13,800 years ago. I would suggest we give them a few generations to get settled in before they start leaving evidence for us to find.