Moons

A moon is a unit of time that is equivalent to one-thirtieth of each cycle of the moon but that has "no human equivalent." Moons are used to measure time in multiple My Little Pony Friendship is Magic episodes and in My Little Pony Equestria Girls.

Development
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(s)
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; in Applebuck Season, Applejack refers to the gathering of Apple family members in Friendship is Magic, part 1 as an Apple family reunion.

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 IDW comics' Reflections Part 1 page 4, Fluttershy states that this magical mirror only works once during each cycle of the moon.

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 Pinkie Apple Pie, Granny Smith states that a Ponyville snail can hibernate for up to forty-eight moons.

In Trade Ya!, Fluttershy states that training an orthros takes a long enough time that her coming and staying with S04E22 Unnamed Pegasus Mare #1 and an orthros in Manehattan until the orthros is trained will cause her to probably be gone from Ponyville for "I don't even know how many moons".