A moon is a unit of time used in multiple My Little Pony Friendship is Magic episodes, in My Little Pony Equestria Girls, and in the IDW comics. The length of time it represents is not determined, and when Friendship is Magic and Equestria Girls director Jayson Thiessen was pressed for its measure, he said it has "no human equivalent."
In early 2014, Ishi Rudell was asked "Since the portal to EQG land only opens every 30 moons, how long would you say a moon cycle in Equestria is?" and answered "Hmmm that's a question better left for Jayson Thiessen. I'd say it's roughly around the same as the human world, but then again, A moon rotation would vary depending on how big or small the planet it was rotating around?", and later in late April 2014, Jayson Thiessen was asked "how long is a 'moon' in MLP? Like, the Portal in EQG opens every 30 moons" and answered "it's a unit of time with no human equivalent."
Depiction in the series
In Apple Family Reunion, Granny Smith states that the Apple family has been hosting Apple family reunions at Sweet Apple Acres every one hundred moons since they first planted roots in Ponyville; a present-day Apple family reunion—as identified by Applejack in Applebuck Season—is shown in Friendship is Magic, part 1, and multiple past Apple family reunions and another present-day Apple family reunion are shown in Apple Family Reunion.
In Princess Twilight Sparkle - Part 2, Discord states that his plunderseeds should have stolen the magic from the Tree of Harmony and captured Princess Celestia and Princess Luna thousands of moons ago but had been being kept from growing up big and strong by the tree's magic.
In Party Pooped, Twilight Sparkle states that Equestria and Yakyakistan have not opened their borders for hundreds of moons. Twilight and Prince Rutherford of Yakyakistan also establish friendship relations "for a thousand moons".
In Every Little Thing She Does, Spike jokingly estimates that Twilight's presentation at the School for Gifted Unicorns will last twenty moons, and Applejack brings eighty moons' worth of photo albums to the castle for scrapbooking with Starlight Glimmer.
Depiction in film
In both My Little Pony Equestria Girls and its screenplay novelization My Little Pony: Equestria Girls: Through the Mirror, Princess Luna states that the Crystal Mirror gateway to the human world opens once every thirty moons and closes on the third day when the moon reaches its peak in the night sky.
In the audio commentary for the DVD and Blu-ray release of My Little Pony Equestria Girls: Rainbow Rocks, when asked "how many moons after" the first film have passed, Meghan McCarthy states, "Six. It is six moons."
My Little Pony: Friends Forever Issue #4 uses the term on page 21, where Twilight states the Crystal Empire disappeared for a thousand moons; and on page 5 and page 19, where Shining Armor states that King Sombra's rule of the Crystal Empire was hundreds of moons ago.
In Princess Celestia and the Summer of Royal Waves, moons are mentioned in multiple chapters. Narration in chapter 1, "Sunrise Over Canterlot", states that Celestia "had hundreds of moons of experience dealing with crises under her crown". She says to Duchess Diamond Waves in chapter 3, "A Princess Abroad", that it had been fourteen moons since they last saw each other. In chapter 4, "Well-Behaved Unicorns", she says the last time she heard about the Time Glass was over five hundred moons ago. In chapter 13, "Attack on Monacolt", Diamond Waves states that Celestia is a thousand moons old.
In Princess Luna and the Festival of the Winter Moon, moons are mentioned in several chapters. Narration in chapter 1, "The Glorious Sun", states that Princess Luna "transformed back into her true self some moons ago". Narration in chapter 9, "A Call to Ponyville", states that she "responded to a call to Smokey Mountain many moons ago, moons after she'd settled the dispute between the bat ponies and the mountain dragons". According to chapters 10 and 14, Tarax Hippos had not been seen in Equestria for hundreds of moons.
In Starlight Glimmer and the Secret Suite, Rainbow Dash calls Ponyville Game Night "one of the most awesome nights in the entire moon," making this the first instance of "moon" being used as a singular unit.
The first use of the term "moons" was in an April 2012 advertisement for the then-upcoming Royal Wedding, quoting Pinkie Pie: "This wedding will have the best reception in 100 moons! 1,000 moons! No, 100,000 moons!"