Pinkie Pie has a strange tale to tell Twilight upon discovering a very strange coin among Pinkie's possessions.

Filly Pinkie Pie wonders why Princess Celestia is sad all the time, even on her birthday. Granny Pie tells her it's because long ago the princess had to destroy her sister Luna in order to save the world and no amount of time has lessened her sorrow. Little Pinkie Pie decides she is going to make Celestia happy with the best birthday present ever! Now all she has to do is build a time machine and go back and change the past...

Posted on EQD way back in 8/7/11 EQD For your Birthday, Princess.