So did only one year pass in-universe?

Not that I’ll deny it if it’s true, but a lot of stuff happened in the show for it to have all been condensed into just one year, especially if you consider the IDW comics to be canon.

Apparently so.  
Perhaps years are longer in Equestria?

Even so, there have been A) Mutliple season-specific episodes and B) Many things from early episodes carrying on to later ones. Not that it’s too big of a deal, but if we make a timeline mixing and condensing all three Season’s worth of episodes into one year (All summer, fall, and winter episodes would happen before Winter Wrap-Up, for example) then I’m afraid that contradictions would start popping up.
 
Unless Luna wasn’t “ready” to show up for the Summer Sun Celebration after one year and it’s really been two years. But then we’re starting to go into over-analyzing-a-TV-Y-show territory.

@Lyra_Heartstrings  
Seriously? Have you never encountered cartoon time before? Frankly, I’m actually surprised that the show is granting that even an entire year went by.
 
And yes, fitting a television show onto a strict timeline is going to be contradictory. That isn’t a surprise, since the show never even pretended that it cared about the passings of seasons.
 
@PaskaNaakka  
Time doesn’t pass in cartoons. Or comics books. Or video games. Or some novels. Or some movies. Or music. Or…
 
It seems like time passing is something that fictional works can overlook…

@Sjogre  
How long has Ash been 10?  
And who can forget DBZ’s infamous “five minutes”.

@soundtea  
To quote someone from another topic (which was ironically directed at me):
 
@Grieffon  

@Psyga315  
How long was Calvin of Calvin & Hobbes in the same grade?

@Sjogre  
I wouldn’t know, for I hadn’t read Calvin & Hobbes. But I see where you’re coming from, and don’t you worry, I’m supporting this “one year” theory with those infamous anime examples as my proof as well. The quote I linked was just a joke I did.

@Lyra_Heartstrings  
Actually, did a lot of stuff happen? Compared to three hundred and sixty five days?
 
There was sixty-five episodes in the first three seasons, and the majority took place in less than a single day. You’d only have to go with an average of two episodes per week to fit everything and have time to spare. A year is enough time for everything that we saw to happen along with plenty of off screen events, although fitting everything into a sensible time line is probably a lost cause.

Wait… so how does that equate to the thirty moons bullcrap from Equestria Girls? Or the 100 moons from the family reunion episode?

@Psyga315  
I always thought that hey used moons instead of a proper measurement of time so that they wouldn’t have to explain how long they take.
 
I seriously doubt that the writers bothered to do the math in either case.

One year hasn’t passed in Equestria. It’s been at least two years. Pinkie Pie’s Birthday has been mentioned twice.

@Battle GT  
Pinkie has only had at least one birthday, and Celestia’s talk about the Summer Sun Celebration only works if this is the first one since Luna’s return.

MLP plays too fast and loose with time and seasons to try to figure out how much time has passed in universe since episode one.

@kleptomage  
With only a bare handful of exceptions, I’d agree. We’ve explicitly seen Spike’s first birthday in Ponyville, we’ve seen Princess Luna’s first Nightmare Night since her return, and, most importantly, this episode was the first Summer Sun Celebration since Luna’s return, since it was the first that Princess Celestia wasn’t be ashamed about.
 
So, season four is exactly one year since the start of the series. After that, who knows?

@Sjogre  
You’re right, the Summer Sun Celebration had to be the first after Princess Luna’s return. It doesn’t seem right though that three seasons and two episodes would take only one year. It doesn’t seem right either that if you assume the Summer Sun Celebration is on the solstice and Nightmare Night on October 31, that in about four months nobody told Princess Luna to speak the modern dialect and not shout at ponies. Then there is the issue of Fall Weather Friends coming after Winter Wrap Up.
 
The time scale is one of those issues I try not to think about too hard.

@kleptomage  
One of the advantages of fiction is that time is mutable. Sure, it means that a calendar time line ends up a mess, but that’s less important than telling a fun story.
 
Luna’s problems with the modern world make less and less sense the more time has passed. If anything, scrunching the series into a shorter period helps the characters.

It’s been at least two years; we’ve had two winters, and we know from Luna Eclipsed that Luna spent her first year secluded somewhere, given she didn’t know any of the new customs or even how people spoke now, so she probably just skipped the first Summer Sun Celebration after her return.

@The Smiling Pony
 
CMC confirmed for lifelong blankflanks.

@The Smiling Pony  
Luna Eclipsed makes less sense the more time has passed, and we know that the seasonal episodes couldn’t have occurred in order for the straightforward reason that that the fall episodes came after the winter episodes. Luna made no mention of spending any time in seclusion.
 
While I don’t think that making a coherent calendar timeline is worth the effort, and would require some contradictions, we know that a calendar timeline would require rearranging the episodes. Putting the two winter episodes next to each other isn’t a stretch.

@Sjogre  
No, she doesn’t mention seclusion or anything, but I take that as an implication. AFAIK it’s been stated that some S1 episodes were aired outside of the production or story order, which is why I don’t look too much into FWF being after WWU.
 
Looking into the seasons as a whole, we know that S1 starts in Summer, was a Winter and Spring. That means that at least half a year has passed before Luna Eclipsed. We then have another Winter. So that’s at least a year and a half before S2 ends.
 
It’s not obscene to imagine that the latter half of S2 and all of S3 take place over half a year until reaching Summer again, which makes for two years.

Also someone could, like, ask the staff or something.

It was a summer sun celebration. There’s nothing pointing to it being the very next one.
 
We know Luna took a while to recover, so she might not have been at the last one, or even a couple. Even still, there’s no reason Celestia couldn’t give that speech more than once.
 
If nothing else, being Twilight’s first time there as princess makes it worth the extra attention drawn to being all together again.
 
People brought up this same confusion with Nightmare Night, and Winter Wrap Up, and the Gala. Don’t stress yourself thinking about this. Just because we don’t see every day of their lives doesn’t mean it didn’t happen.

@The Smiling Pony  
Four months of seclusion would fit with Luna’s lack of experience with the modern world. A year and change, not so much.
 
While episodes in both seasons were aired out of production order, production order still places a fall episode after the winter episode in the first two seasons. Storylines have been somewhere between minimal and nonexistent, so shuffling episodes around without disrupting that is not a problem.
 
Spike’s birthday is required to part of the first year in Ponyville, meaning that all of the season one had to occur within a single year. Additionally, Luna Eclipsed took place in the fall, judging by what had been harvested, so Spike’s first birthday in Ponyville would have been more than a year after moving there if you go by the seasons shown. The seasons contradict the time that has been said to pass, and I’d go with explicit time passed over calculations passed on seasonal episodes.
 
@Frostwolf123  
From Celestia’s speech, this is the first one that she has ever looked forward to one. It is also the first time that she’s ever mentioned it to Twilight, and given that Twilight helps to set up the Summer Sun Celebrations, not hearing that before would be decidedly odd.
 
So, yeah, we’re given reason to believe that it is the very next one.
 
Don’t act like the series hasn’t presented itself as covering a short time span before. The end of the third season outright said that Twilight has been in Ponyville only a short time.

Well there were two holiday winter episodes, possibly a third this season, so at least two years, not one. I suspect two and a half if we stick in the IDW comics, which is a pretty realistic timeframe for the amount of events we got.