The title of “After Twenty Years” is ironic because two old friends are reuniting after a long time, but they are now on opposite sides of the law.
When Bob sees Jimmy, he does not recognize him. They were young men when they last saw each other. He explains why he is waiting to Jimmy, thinking he’s just a cop.
It's an appointment made twenty years ago. Sounds a little funny to you, doesn't it?
The ironic element of this is that keeping an appointment from so long ago does make you a good friend. It is an amazing and sweet gesture, especially for Bob. Bob had to come all the way from Chicago to keep the appointment, Jimmy stayed in New York.
Jimmy Wells [was] my best chum, and the finest chap in the world. He and I were raised here in New York, just like two brothers, together.
The ironic thing about the title is also that if Bob had not come back, after twenty years of being away, he would never have been arrested. He was safe in Chicago. He kept an appointment with the one person who would recognize him and could turn him in.
Yet although twenty years is long enough to change "a good man into a bad one" it is not long enough for either good friend to turn into a bad friend. Jimmy does turn Bob in, but he doesn’t have the heart to do it himself. He has another cop arrest Bob.