Seems we're getting a lot of these lately, eh? I guess we're just stepping it up in trying to keep our database as consistent as possible...

You can read the full policy in the FAQ here:
http://www.mangaupdates.com/faq.html?cid=7#item52

Basically, the year to enter is the year when the first chapter was published, whether that be in a magazine or a volume.

If it is a volume compilation of oneshots, enter the year the volume was actually published (and not the individual oneshots).

A just ruler amongst tyrants