How many times have you been told you should present for X minutes, then give the audience Y minutes to digest what you presented? In an introductory Communication class, I shared with my students they should present for no more than 10 to 15 minutes at a time, then provide the audience with an opportunity to interact with the information shared, but I was not ready for this question from a student: "How do you usually keep track of the time without always looking at your watch?"
I thought, "Hmm ... That's a valid question!" I had not realized up until that point there is a number of techniques all connected to increments of time that I subconsciously use in an effort to give audience members the experience I would want to have if I was in their shoes, and I have now consciously listed them here ...
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 has yet to arrive and you must wait for his/her arrival before you begin. Don't you worry for a second. You've got this.