If you want to know when the ice cream man comes by, you are going to need to ask him. Since he's an independent driver of an ice cream truck, he may not even have any kind of set schedule.
I'm assuming you're talking about something like the guy that drives a big van or box truck that's designed to sell frozen treats out of the side and plays music. The next time you DO catch the ice-cream man, just ask him if he always comes at the same time and if so, when that time is. That's also assuming it's always the same truck and driver.
This is a helpful suggestion incase nobody else can provide a definite and honest answer.
Even if the guy has a basic route and schedule, he'll probably never come at exactly the same time, because traffic and fluctuating business will throw it off a little anyways. That is, unless he just drives to only one specific location and parks for a specific length of time.
Good luck. :^D [Yes, ice-cream is tastey.]