Another detail about dates people often forget or don't know (though it's not usually important), is that a "date" can be 50 hours long. Let's say you want to have some sale to occur all day on Christmas, midnight to midnight in your customer's local time.

For easier math let's say your business is in GMT tz. People in the Line Islands (Pacific/Kiritimati) will have access to the sale 14 hours before yourself. Then the sale-time occurs in GMT, 24 hours pass, and the sale ends in GMT. But people in American Somoa (Pacific/Midway) will still have the sale active for 11 hours. (Technically someone using satellite internet in the open ocean east of Samoa has 12 hours, but the UTC+12 timezone is completely uninhabited).

Such a sale is kind of a farfetched example, but I've encountered this issue when trying to do analytics reports looking at user data that happens on specific global holidays.