Note: This timeline is not done and probably inaccurate or biased in some parts. Leave some suggestions in the comments!

1809: Abraham Lincoln is born.

1865: Abraham Lincoln dies.

1,000+ years before the pilot: The Great Mushroom War happens (The Great Mushroom War might have happened around the late 20th-early 21st century, most likely the late 20th (1980s-1999), because VHS is sitll around in "Video Makers".)

Sometime after the Great Mushroom War: The Rainicorn-Dog Wars happens. (Maybe before the GMW?)

1,000 years before the pilot: Marceline is born.

500 years before the pilot: Old Lady Princess is born.

18 years before the pilot: Princess Bubblegum is born

15 years before the pilot: Lumpy Space Princess is born.

13 years before the pilot: Finn is born.

Sometime after Finn was born: Finn was abandoned. Joshua and Margaret adopt him.

Sometime after Jake's birth but before the pilot: Joshua and Margaret die.

3 years before the pilot: Jake is born (You might be wondering how I came up with that. Well, using the WebMD Dog Age Calculator, it says that all kinds of dogs (Small, Medium, and Large) are 28 when they're 3 in human years. Jake is 28 in dog years.)

During Mystery Train: Finn turns 13.