I would have to agree that the appointment is uncertain. As the narrator describes it;
And in the door of the hardware store the man who had come a thousand miles to fill an appointment, uncertain almost to absurdity, with the friend of his youth, smoked his cigar and waited.
The author makes his story dramatic by raising the question of whether Jimmy Wells will actually keep the appointment. No doubt both men were drinking when they agreed to meet at this location in twenty years. Jimmy could have forgotten all about it, even if he had made the agreement in good faith. He might not even be alive after twenty years. A lot of things can change in that time. O. Henry symbolizes the way things change by showing that "Silky" Bob came expecting to find "Big Joe" Brady's restaurant and instead is standing in the darkened doorway of a hardware store which has been there for five years.
Jimmy may have thought about keeping the appointment but decided that his friend probably wouldn't keep it because he had gone clear out west. If his friend was still in the West, would he travel a thousand miles or more--perhaps as far as three thousand miles if he got as far as California--to keep an appointment when he wasn't even sure his friend would be there?
The two friends were only eighteen and twenty years old, respectively, when they made that appointment. They may have felt differently about each other and about life in general at that time. No doubt "Silky" Bob has become hardened by his criminal career in the West, and he has no reason to believe that twenty years have not had a comparable effect on his old friend. In fact, both of them have changed considerably by the time they actually do meet, so it is odd that they should have felt obliged to keep that fanciful appointment they made while intoxicated and sentimental in their late adolescence.
The appointment they made to meet again after twenty years was exactly the sort of thing men do while in their cups. Liquor either makes men affectionate, sentimental, or belligerent. And it can also make them forgetful. It isn't hard to believe that one man would keep the appointment, but it is hard to believe that both of them would keep it, or even remember it.
"Silky" Bob himself is not really sure that Jimmy will show up at the appointed time and place. He only plans to wait for a half-hour past ten o'clock.
"Hope your friend comes around all right. Going to call time on him sharp?"
"I should say not!" said the other. "I'll give him half an hour at least. If Jimmy is alive on earth he'll be here by that time. So long, officer."
Bob acknowledges the possibility that Jimmy might not even be alive. He does not understand the ominous implication in the policeman's question, "Going to call time on him sharp?" The cop (Jimmy himself) wants to know if he will have time to contact headquarters and have a plain clothes detective come to arrest his old friend. Bob has pulled out his ornate watch and stated that the time is three minutes to ten, so Jimmy has at least a full half-hour to summon a backup.
O. Henry's stories are usually full of irony. The irony in "After Twenty Years" is that Jimmy hasn't changed in being steadfast in keeping his promises or in his friendship but that he has changed in every other way. Everything changes and everybody changes over time. The changes are typically slow and gradual but often astonishing to anybody who has been away for many years.