The Crimson Binome
Hero
If you're following an official setting, then most events have dates attached. The Eberron campaign setting describes not just a world with a particular history, but it also takes place in a particular year. Likewise with Forgotten Realms in any given edition - The year is 20XX.Then they all roll up new 1st level characters and start the Prince of the Apocalypse, with no mention or reference to what just happened with the whole Tiamat thing. That is, if the PC's, for whatever reason, go to Greenest (is that what it's called?), it will be perfectly fine. It was never attacked by a dragon and dragon worshipers, and Tiamat never showed up to be defeated. In short, that "campaign" was a "bubble story" with no consequences or lasting effects to continue an ongoing "actual" DM's campaign.
That means everything that happens in that setting is taking place during the same time frame. If you're running a second campaign within that same setting, then part of that setting involves a return to the year that setting represents. The other campaign hasn't happened yet.
Unless you chose to advance the timeline, of course, but that's all on you. Imagine if you're new in town, and you found a group that works for you, and they're even playing in your favorite setting, but then you show up to find out that their version of that setting is way different from what you expected. You'd probably still play with them, but it would take a bit to get adjusted. It wouldn't feel quite right, especially since you weren't there to watch all of those changes happen to the world.
Personally, I put the blame on the shift from homebrew settings into official settings (which DMs are less likely to want to disrupt, for fear of making their books obsolete). Some of it also probably comes from the shift from long campaigns (multi-year) to short campaigns (try to wrap up everything 1-20 in one year).