The number of days shown on your profile is correct; Area 51 is under-reporting.
All Stack Exchange times are in UTC. We received the email on what would have been late Tuesday UTC. Assuming you created your account promptly (looks like you and I both did), you got credit for those couple hours of Tuesday. Now it is Sunday (UTC), so six days inclusive (which your profile now shows).
However, Area 51 is (now) showing 4 days. Maybe Area 51 didn't count Tuesday and/or isn't on UTC, but that would only explain being off by one day. So it sounds like Area 51 is being overly conservative, for some reason unknown to me.