Can someone help me figure something out?

I think it may be months, I suck at time mathematics but I'll try this. Every 100 moons, means every 100 months, which is about 8 years, so they probably hold the reunion every 8 years. They never did specify which reunion was the one where アップルジャック, applejack was a foal, so it could have been any time before. At the very least, it could have been 2 reunions 前 meaning アップルジャック, applejack would probably be 18 または 19.

Actually, if we went によって full moons, it would be 8 years, 4 months.

We get 12 full moons a year, times 8 is only 96. On the 9th year, we would be 1/3 of the way through the year, または 4 months.

So they hold the reunion every 8 years, 4 months.

It might mean months または years

または maybe it means full moons

Possibly weeks?

I'm going to go with the flow and say months. If it were weeks, there probably wouldn't be much stuff done outside of the reunion. If it were years, we wouldn't see any of the ponies we saw during the episode. That's my theory.