Thread:Greenny/@comment-263488-20130823183428/@comment-3474542-20130824062727

Time limited items, like: Level 1 trees, cows, horses, buildings, gnomes, and decorations (among other things) have a last available date.

Things are permanently available, such as: calves, foals, chickens, level 2 trees, permanent market items, or things that come from mystery babies do not have a "last_available" date.

Last available is an optional parameter, for items that are actually have a date that they are no longer available (for example the 1-Year Balloon). It is not for items that are either never or sporadically available in the market, but are permanently available elsewhere (like the Gold Gnome).