Why does the police officer need to know if the man would wait for his friend or "call time on him sharp"?

O. Henry wrote with irony, and the ironic aspect of this story is that the police officer is Bob’s old friend, the very man he is anxious to meet. When Bob lit up his cigar, the police officer, Jim, recognized him from a wanted poster as Silky Bob, a person wanted in Chicago. He knew he had to arrest him, but he couldn’t do it himself. His friendship was too strong. He had to get someone else to arrest him, so he went around the corner and contacted a plain clothes officer to handle the arrest. Jim needed to know that Bob would stay there waiting until he could contact the plain clothes officer. He didn’t want him to leave. He asked him how long he planned to wait for his friend. Would he leave at 10 sharp, or would he give his friend a few extra minutes. Bob replied,
               

“I’ll give him a half an hour at least.”  (pg 2)  

That was enough time for the police officer to contact the plain clothes officer and have Bob arrested.