Boardwalk's own timeline was a bit a messy (for example, they have Mabel being born after Gillian!!!) so here's a version that leads up to Clara and Nucky arriving in Chicago. There's also a few spoilers and previously unwritten background information contained here.

Nucky Thompson and Mabel Jeffries born

Eli Thompson born

Arnold Rothstein born.

Gillian Darmody born June 15th

Margaret Rohan born in Ireland.

Mabel Jeffries graduates from college and begins teaching. Nucky is appointed Deputy Sheriff.

Richard and Emma Harrow born December 13th in Wisconsin.

Nucky Thompson and Mabel Jeffries marry.

Gillian Darmody meets Nucky Thompson.

Mabel Thompson suffers first miscarriage.

Jimmy Darmody conceived in May.

Nucky Thompson becomes Sheriff.

Clara Thompson conceived in November.

Salvatore (Charlie/Lucky) Luciano born in November in Sicily.

James Edison Darmody born January 31st.

Clara Susan Thompson born August 13th.

Al Capone born in February.

Meyer Lansky born on July 4th.

Eli and June Thompson marry.

Nucky Thompson becomes treasurer of Atlantic County.

Benny (Bugsy) Siegel is born in February.

Enoch Thompson Junior is born and dies. His mother, Mabel Thompson, dies a month later. Clara finds her mother's body.

Nucky and Clara move into a few rooms at the brand new Ritz-Carlton.

Eli and June Thompson have their first child, Willie.

Margaret Rohan immigrates to the United States.

Clara Thompson and Jimmy Darmody contract malaria. Nucky nurses them both.

Arnold Rothstein marries Carolyn Greene.

Halley's Comet visible; Richard Harrow watches from Wisconsin.

Jimmy convinces the entire seventh grade at Our Lady Star of the Sea that Father O'Connor is having a secret affair with Sister Mary Agnes.

Anna Citron born.

During a Nucky's Nocturne party a drunk guest wanders into Clara's room. Nucky has her private, locked hallway constructed and buys her a gramophone.

The Commodore Louis Kaestner, Jimmy's biological father, goes to prison.

Nucky enrolls Clara at Foxcroft Boarding School in Virginia.

The Harrows acquire a new puppy, Sampson.

Richard Harrow graduates from high school and begins working at the Plover hardware store.

Eddie Kessler begins working for Nucky Thompson.

Margaret Rohan marries Hans Schroeder.

March: Teddy Schroeder born.

June 28th: World War I begins in Europe.

Summer: Eddie Kessler teaches Jimmy Darmody and Clara Thompson how to drive.

Late Summer: Clara refuses to return to boarding school and joins Jimmy at Atlantic City High School.

May

Emily Schroeder is born.

Clara Thompson and Jimmy Darmody graduate from high school.

September

Clara begins college at Bryn Mawr; Jimmy starts classes at Princeton.

Jimmy meets Angela. They conceive their son a few weeks later.

November

Gillian Darmody visits Princeton.

Angela tells Jimmy she's pregnant.

Jimmy enlists in the Army.

Jimmy goes to Bryn Mawr to tell Clara he's enlisted.

Jimmy leaves for Camp Grant in Illinois for basic training.

December

Jimmy writes Clara to tell her about Angela and the baby.

Clara leaves Bryn Mawr, travels to Princeton, finds Angela. Clara takes Angela home with her to Atlantic City.

Clara tells her father about Angela, and also informs him she's dropped out of school and wants to go to France to work. Eventually, he agrees to let her move to New York with Angela and work for the War Department there. Nucky tells her he will support Angela as long as Clara doesn't try to get another assignment to France.

February: Jimmy spends his last days before shipping out with Clara in New York. He makes her promise not to tell Angela, Nucky, or Gillian.

April 2nd: The United States declares war on Germany.

June 5th: Richard is one of the first American men drafted for service.

July 6th: Thompson 'Tommy' Darmody is born.

July 8th: Richard begins basic training at Camp Grant.

September 5th: Commodore Louis Kaestner leaves prison.

October 10th: Richard ships out. He's sent first to the First Army School of Sniping, Observation, and Scouting at Linghem in France.

June: Clara receives an assignment from the War Department which requires she leave New York. Angela and Tommy move to Atlantic City.

Late September: Richard Harrow is injured when Germans shell the tree where his sniper's nest is located.

October: A German grenade injuries Jimmy's leg, which is only saved with experimental surgery.

November 11th: World War I ends.

Late November: 

Richard returns to the U.S., and has multiple surgeries at U.S. Army General Hospital No. 1, at Williamsbridge in New York City.

Clara returns to Atlantic City. Her emotional state worries Angela and Eli. Nucky broaches the idea of her marrying the son of a political acquaintance.

December 1st: Clara moves to D.C. and begins working for the actuarial office of the War Department, temporarily located at the Museum of Natural History.

December 10th: Richard's mother dies of influenza during the pandemic.

January: Jimmy arrives at Walter Reed Hospital. Only Clara knows where he is.

March: Clara, suffering with insomnia, begins writing and trying sell articles at night after work.

April 20th: Clara goes home for Easter. During Nucky's Easter party, where he hosts the Yacht Club among other local dignitaries, Jackson Parkhurst so upsets Clara with his talk of war profiteering that she considers stabbing him with her steak knife.

May: Richard is sent home to Wisconsin. Emma cares for him as he recovers. Richard realizes he has no feelings left for her.

October: Richard leaves Wisconsin for Chicago. Within weeks he's been contracted to perform his first civilian hit.

December

Clara's War Department job ends and she returns to Atlantic City. Nucky throws a Welcome Home party and introduces her to Darcy Blaine, and tells her he needs her to serve as his hostess.

Jimmy has disappeared from Walter Reed. Nucky asks Clara to marry Darcy Blaine as her Christmas present to him. Clara agrees. Clara's friend Rose Grenville reaches out to Angela out of concern for Clara.

Jimmy returns to Atlantic City on New's Year Eve.

January

Prohibition takes effect.

Jimmy meets Al Capone and they ambush Rothstein's men.

Leander Whitlock witnesses Darcy bending Clara's fingers back after Darcy sees her talking to Jimmy.

February: Nucky banishes Jimmy to Chicago, where he begins a relationship with Pearl.

March

Clara sells a story to Good Housekeeping.

Pearl is attacked and eventually commits suicide.

April

Clara and Margaret meet for the first time.

Charlie Luciano begins an affair with Gillian Darmody, believing her to be Jimmy's wife.

May

Jimmy and Richard meet at the Veteran's hospital.

Rose Grenville's parents write Clara and ask them to come stay with them out of worry over her letters and her clear unhappiness at the prospect of marrying Darcy Blaine.

Clara begins piecing together Jimmy's location and finally convinces her father to take her with him to Chicago for the Republican Convention.