No thrills. No frills. The message of the day is begin a presentation on time, and complete it on time as it shows respect for the audience's time. When you fail to do either or both, then the message you send to your audience is "I do not care about your time." Plain and simple.
Your audience members who have shown respect by arriving early or on time are due the same respect. You may ask what to do when you have a glitch with the technology that causes a delay or what to do if a person in a key position relative to the audience members is missing and you must wait for his/her arrival before you begin. Don't you worry for a second. You've got this.
If that or any other instance is the case, then let the audience know at the official start time and proceed to everyone in a meaningful exercise. A quick statement to this effect works: "Thank you for coming today. We are fixing a technical glitch/we do not want to begin without your division chairperson. We will get started shortly. In the meantime, please write down what you want to get out of today's session, then discuss it with the colleague sitting next to you."
Additionally, ensure you avoid using phrases such as "If I had more time, I would ..." or "For those of you who can remain a few minutes after the presentation, I'll share ...." It suggests to the audience that it's the organizer's or the audience's fault you do not have more time, and that one of them was careless in scheduling "such a short amount of time" for you. Additionally, it is unfair to penalize those - positioning them to miss out on additional details - for operating in accordance with the advertised start and end times of the presentation.
Audience participation can become so robust until you may find yourself having to jettison some material. Know before your presentation and during your practice time what information can be left out so you are prepared if you find yourself in a position where you need to omit material in the interest of ending on time. That information should be anything that would not interfere with the audience being able to understand your message and or act as a result of having heard your presentation. If you have to omit any information, simply omit it without any mention to the audience, and keep it moving. You are the only one who will know you did not cover it. And the bonus is ... you might have a piece of material that positions you to schedule a follow-up performance!
Finally, the audience tends to mentally (and sometimes physically) check-out when you approach the final minutes of your presentation. Do you remember when you were in a face-to-face class? You watched the clock, and the closer it got to the last minutes of the class, the less engaged you were with what the teacher said. The same goes for any other audience. Ensure you plan to always include a call to action for your audience. Now that they have heard your presentation, what do they do now? Factor in ample time to answer that question for both you and your listeners.
The takeaway? Start on time; end on time.