ULX has two types - grouped and non-grouped.
Non grouped are zero based. Will show up at whatever time specified, individually.
Two adverts
Advert 1 at 60 seconds
advert 2 at 65 seconds
Second time, 1 will show up at 120, 2 will show up at 130
Grouped, 2nd add will always show up constant <x> after first>.
They're consecutive.