You know, in the years I've been RP'ing here, I don't think the "when" of Archet's burning has ever come up. It probably has, just not when I've been around. The issue with the starter instances is that we've all done them, whether or not it made any sense for our characters to be there. So, for example, my man of Gondor was passing through Bree-land and happened to be around when Archet was attacked... it wasn't a major point in his storyline, but as my main I try to work him into everything. To my Dunedain ranger (or as close as I can approximate), Archet was more important because he lived in the north. On the flip side, my elf simply ignores the starter instance in Edhelion, because in the timeline I have for him, he just wasn't there.
Now being from Archet, the attack by the brigands is going to be a central part of *your* story. Therefore it exists when you want it to. If you say I was in Archet when it was destroyed last week and in my timeline it was three months ago, I personally would just let it go. You might get the occasional person that would try and call you out, but not only would it be rare, it would also be pretty rude. It just part of the MMO in our RPG.
Time is confusing in LotRO, because it doesn't actually move. Different places exist as bubbles in time, though with landscape instancing they've started to advance time in certain areas; the Fellowship moving forward, for example.
All that said, if you want to get really technical... our game begins on the night of September 23rd, 3018 (in the Hobbit intro you see Frodo, Sam, and Pippin on their way to Woodhall). The last book related thing we see is Eomer being placed under house arrest which occurs on February 30th, 3019 (Vol. 3, Book 9). Which means that everything we've done so far has occurred in a span of five months. If you want to get really weirded out by the passage of time the Battle of Pelennor Fields is fifteen days away; the ring gets destroyed in twenty-five days. Think of how much we'll actually be doing between now and then!