Sometimes, you see the agenda at some conferences: “We start the conference at 7 PM”.
And you go there, and it actually starts a bit later, 10 or 15 minutes later.
I used to organize a series of events, and when we started announcing, “We will start at 7 PM,” but we actually started at 7:10 – 7:15 PM (so 10/15 minutes later), people would get accustomed to this and would start arriving late.
The solution? Start on time; people will appreciate this and be there at 7 PM sharp.
Also, if some people are late, consider starting with something less important. Introductions, general overview, a small chat with the audience, anything is better than waiting for 10/15 minutes.
When it comes to ending a conference, meeting, seminar, etc., people tend to be very strict—”We will definitely end the event at 20:30 PM sharp.” I would consider applying the same strictness to starting the event.
If you need to have a prior time to the event for registration, announce it like this: “The event will start at 7 PM sharp, but since 6:30 PM, you can come to the location and do networking, register, and interact with us”.