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 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.