So even though you know you joined on Friday the 30th the system shows it as the 31st.
Not that big of deal, most don't notice it unless looking at the dates of disruptors.
To see yours, or anyone's "joined on date" all you need to do is hold your mouse over the screenname (the one after "by") and a box will show the date.(on mine it is a yellow box but it might differ for others.)
Can also click on the names and go to the profile page and see the date there and any other info the user may have added.
We call them "lurkers". :`)