All leagues cover a certain period of time; the weekly leagues for example, cover 1 calendar week, ditto for the monthly and yearly league.
As such, those leagues must include their starting date in the URL, to distinguish them from all the other weeks or months.
The all-time league is no different from the others, in that respect. Except that the time period it covers started on 2008-07-31, the date Stack Overflow started. The site is just reusing the same code to produce URLs as for the other per-site leagues, to keep things simple.
The date itself is otherwise normalised; if you alter the weekly or monthly or yearly league dates, the code uses the next earlier date; if you use 2013-03-31 for the yearly league, for example, the site is smart enough to then pick the 2013 league and all further links use 2013-01-01. The same happens again for the overall league; put in a bogus date and the site just uses the 1 and only starting date it has instead.
If you look at the leagues for different sites, you'll see that they all too have a starting date in the all-time leagues; Superuser started on 2009-07-14, for example, while Home Improvement started 2010-07-21.