In the list of available webinar times for registrants, show an option that starts at the next 15-minute mark (just in time for them) from whenever the person is viewing the registration page. This is in addition to the other pre-scheduled time options that are set up ahead of time. As an example, if the evergreen webinar is set up to be available at 1pm everyday PLUS the JIT option, and the reg page is set to generate the next 2 available times, and they are viewing the reg page at 9:18am, they would see a 9:30am start time and a 1pm start time as their options. Someone else viewing the reg page at 9:31am would see 9:45am and 1pm options.