On that first day, the site is listed as having been in beta for "0 days."

But your 'visited days' are counted from day one. So, when you visit the site for the first time, you already have "visited 1 consecutive day(s)." 

So how is the count off by two?

The day count in 'consecutive days' is based on the UTC timezone. So here in the US (for example), you can visit again late in the day and have started your second consecutive day (i.e. "visited 2 consecutive days") ... and still, the site is in beta on it's first day -- "in beta 0 days".