The changeover to the Gregorian calendar system occurred as described above only in Roman Catholic countries.
It is assumed that 7-day weeks have formed an uninterrupted sequence since ancient times.
Fling in 2011, Linked In in 2012, tumblr in 2013 and the mother of them all, My Space in, well, we don't quite know. Firstly, the only data in the breach is an incrementing ID (possibly an internal My Space identifier which would enable to date it), an email address, username and one or two passwords.
There's been no information forthcoming from anyone about when this breach actually occurred and there's no explicit indicators in the data dump either (sometimes there are timestamps on account creation or website activity). The passwords are stored as SHA1 hashes of the first 10 characters of the password converted to lowercase.
Let's find some stats on the previous year, for example this cart from Tech Crunch in Jan 2009: Here the ratio of Gmail to Hotmail starts out at about 1:2.5 in Oct 2007 and finishes at about 1:1.5 in Dec 2008.
Now again, this is not to say that as of the end of 2008 My Space would have had 1.5 Hotmail accounts for every 1 Gmail account as they'd already had 6 years of accumulating Hotmail customers and only a couple of years of Gmail customers (at least since they went out of beta).