They have stated (in a dev post on the forum IIRC) that "until the end of summer" means until the end of September. So regardless on when summer ends where, this is what they mean.

They have never said 1.20 would come out by summers end - all they said is that the list in Letter from the Producer, X would be ready at summers end. Seeing as how most of the stuff in that list has either been included in 1.18 and 1.18a, or has been stated to come in 1.19 (materia system, crafting recipe changes, battle algorith changes, chocobos, airships, Ifrit battle) I don't see how 1.20 would be coming out by the end of summer.

And you have to read closely on some of the items on that list. For example the job system items say nothing about implementing a the job system, it only says they will work out the details for it and calculate cost of implementation, meaning by the end of September they will be able to talk a little bit mor eabout it.