pari version and dates

[John Cremona <john.cremona@gmail.com>]

Apologies for a question which is probably very silly.

One of my computers has version 2.5.4 in /usr/bin, specifically
/usr/bin/gp-2.5,  which has a datestamp of 17 Nov 2011, and when it
runs the banner says that it was compiled on that day (presumably by
me as I administer this machine) and the banner ends with Copyright
(C) 2000-2011 The PARI Group.  The version number is 2.5.4 (released).

But on http://pari.math.u-bordeaux.fr/download.html it lists 2.5.4 as
the most recent released version, dated May 2013.

So how come I made a released version 18 months before the release date?

More importantly, should I replace that 2011 version on the system
with a freshly compiled 2.5.4 or would it be identical?  I just built
it and the banner is very similar except that the copyright now ends
2000-2013.

I do not usually install development versions system-wide, but would
do if I receved an authoritative recommendation!

Bonnes vacances a tous -- moi, je vais en Bretagne demain!

John