Time in Equestria

Before I can date the episodes, I need to decide about the year
Their year might be:
1) 13 months of 28 days = 364 days.  If you assume that the Summer Sun Celebration is an  intercalary day, that = 365.  
2) 6 months of 30 days alternating with 6 months of 31 days = 366 days.  Since they control the sun, I see no reason why they would bother with Leap Years.
I'm going with #1, 365 days

Next question
"When does their calendar start?" Logical dates include
Celestia banishes Luna (lot of  fan fics seem to assume this) 
Start of Celestia's reign. IRL, a lot of ancient kingdoms did this.  Means you have to start renumbering every time the ruler dies, but EQ would avoid that problem
Building of Castle of the 2 Sisters. IRL, the ancient Romans dated their calendar from when they reckoned Rome was built (754 BC -they got it wrong).  It probably wouldn't be Canterlot, that wasn't the capital until after Luna's exile.
Creation of the world -if they know it or have a dateable creation myth. Ancient Jewish calendar did that (ICR the conversion factor. No, not The Inquisition) IMO, this makes the most sense because you've got no BC/AD confusion & no nonsense about counting years down. This is irrelevant for my purposes.

For the seasons
I first thought the Summer Sun Celebration (start of Summer) would be June 21 (longest day of the year).   That would put the Running of the Leaves (Autumn) as Sept. 21, the start of Winter on Dec 21 (It was Season 5, Tanks For the Memories but ICR what it was called) & Winter Wrap Up on March 21.  That is, their seasons are the same as ours.

There are 3 days that most fans equate to Earth days.  Nightmare Night = Halloween (Oct 31), no problem (Well, last day of Oct., close enough). Hearth Warming = Christmas (Dec 25), no problem.  Hearts & Hooves Day = Valentine's Day (Feb 14). BIG problem.
It is clearly a Spring day, which means it has to be after Winter Wrap Up.  Big Mac isn't busy, so planting is done.  It must be at least a few days later.  This means 1 of 2 things.
1) Winter Wrap Up is no later than, say Feb 10.  This means either:
1a) Winter in EQ is VERY short. (Probably no longer than needed to create an adequate snow pack to ensure enough ground water). This makes some sense. The reason there are so few Winter episodes is that there isn't much Winter to have episodes IN.  However, in the song Rarity says "90 days of Winter" (technically 91, but that's quibbling) & that's wrong.
OR
1b) The seasons are all cattywampus from Earth.  Summer  starts early May, Autumn starts in August & Winter no later than Nov 10. Still works for Nightmare Night & Hearth's Warming dates
2) Hearts & Hooves day is at least 6 weeks later than Valentine's day, March 22 or later.  About when Easter is on Earth.  

I'm going with #2. Winter Wrap Up is March 21 & Hearts & Hooves Day is later. The song is right about "90 days of Winter"