What's Up With The Stranger Things Timeline?
Plainly, it appears like Stranger Things treats each episode like a new day. Characters continue on their various interlinking threads hour to hour besides, yet when it comes to addressing the actual passage time each opening title is a new dawn, even when everything suggests otherwise. This isn't an overly complex show narrative-wise - everything goes in a linear direction and thanks to its strong cultural influences you have a good idea of what's going on intuitively - but that only makes that this keeps happening more confusing; if you pick up on these moments, you'll spend ages trying to dissect the very simple timeline trying to justify all this confusion.
And the simple answer there is no explanation. We're not meant to think about this too much, rather just let it go and keep going - Joyce was just speaking out of turn and story-wise Eleven needed to travel afar for her arc to work as well as it does. But that handwaving is hard to swallow when everything else in the show is so well constructed; how can you have astutely written teenagers of all description and forget how many days have elapsed in your story?
It's Because of Netflix's Release Strategy
The answer may lie in the Netflix release method. First up, you're only really going to spot this slip-up on binge watch when the specifics of what happened before are fresh in your mind. However, the streaming itself may the root cause too.
Being on Netflix is usually a real source of freedom. There aren't the restrictions of even cable channels and, when it comes to storytelling, creators are given free reign (look at how Hollywood cartoon comedy BoJack Horseman has become the primo cultural dissection of depression). In many ways, this bears fruit for Stranger Things. It can go, well, stranger, and invest time into more out there ideas (Episode Seven will divide audiences regardless of their openness to the timeline). The pacing also benefits from a non-regular release structure, with episodes varying between 45 minutes and 1 hour in length depending on what was deemed suitable by the creatives.
However, Netflix also brings with it a variable consumption method. Some will binge the entire show in one go, some will watch one a week akin to standard viewing habits, while other may scattershot consume bit by bit. Episode-to-episode, this typically leads to a narrative structure that straddles the line between traditional episodic and Golden Age serialized TV; each episode aims to stand by itself so single-serving viewers leave satisfied, yet there's a strong overarching thread to keep bingers going "one more before bed". This is a different approach to your usual TV show and its good and bad effects can be seen across the service's output, perhaps most prominently with House of Cards and the Marvel Netflix shows (when they're hot they're scorching, but can often feel bloated).
The main thing to take from all this is that Netflix creates content to appeal to its audience, on which has an awful lot of data; they fit their shows to you. And, making the fair assumption that the average consumption of a season across all Netflix users is around one episode a night, that leads them to having Stranger Things feel like it's going at a rate of one day per episode: while to Joyce and Will Halloween was just yesterday, to the average viewer it was in fact two days ago and thus the night should be clarified; and Eleven's journey would be completely manageable with a few extra day's time. It bothers us, but won't even be detected by most.
Overall, the discussed effect of being on Netflix is less pronounced with Stranger Things than its contemporaries; each season runs as a complete story where you can dip in and out at any point. However, that "once a day" viewing habit appears to have manifested with these moments of reference.
It's ultimately a quirk and definitely not one that damages the show. Stranger Things 2 is, altogether, a great season of television. It's dramatic, it's funny, it's emotional. It's so damn good it comes closer than perhaps anything before to blurring the lines between the big and small screen. But that makes it a shame that the show has these niggling marks of its creation. Here's hoping that, like how many of the slight problems with Season 1 were dealt with in the new run, any such issues are gone when Stranger Things returns next year.