Plan with no dates

Hi, I am trying to layout an emergency response timeline using OmniPlan. I’m having trouble using the “unspecified dates”. I just want to have 15 minute increment events that start at the unplanned emergency. Let’s say there’s a fire, then it might say Tday + 15minutes: Firetrucks roll out, etc… Have any pointers or tips for doing a timeline with no specific dates or times?