I saw an episode of Cutie Mark Laboratories that covered this & was inspired to go into my thoughts on the matter.

The episode Point Of No Return established that all months are 28 days. 13 x 28 = 364. If you assume that ponies have an intercalary day (perhaps the Summer Sun Celebration) then their year is 365 days.

IMO, either is close enough to an Earth year to use to measure lifespans.

I think the first 3 show seasons cover 1 year because
Holidays

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...that turned down roles that brought fame to others. The internet had a list but IMO they left out a few.
The Wizard Of Oz
It was originally supposed to star W C Fields and Shirley Temple. Shirley's mother didn't want her around Fields (a drunk). Then Fields' health didn't permit him to make the movie.

It was Judy Garland's most famous role. The movie was nominated for 5 Oscars including Best Picture. (Lost to Gone With The Wind)

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