What's the chronological order of the flashback episodes?

Occasionally Person of Interest has episodes that flashback to before the start of the series. These episodes focus on various elements of The Machine and friend's origins and history. They're spread out over the seasons, which makes keeping track of them a bit messy. I'd like to rewatch them in the in-universe chronological order though.

What order are the flashback episodes of Person of Interest in, chronologically?

It'd be especially helpful to include a short description for what story or element each flashback deals with.