Every event has a time zone, and its times are always that zone's clock. A new event takes the zone in Settings; change it for one event with Time zone at the foot of the When editor, where you can search by city.
Guests see the event's own time, with no conversion and no zone shown. A 7 pm dinner in Chicago reads 7 pm to a guest in Boston, so if your guests are spread across zones, say which one in your notes.
Add time sets an end, three hours after the start to begin with. An end after midnight but before 6 am counts as the same night. Add date ends the event on another day, for a weekend away, and Same day goes back.
Guests don't see the end time on the invite; it goes into the calendar entry when they add the event to theirs. With no end, the calendar entry runs three hours, and the event counts as over six hours after it starts.
Without a start time, the invite reads Time to come and the calendar entry is all day. Send an update when you know.
The reminders follow the event's zone too: the day-of reminder at 9 am there, and the automatic nudge between 9 am and 8 pm there.