Globalizing APIs is hard. I’ve posted before about some of the pitfalls of dates and times in APIs in The 5 Laws of API Dates and Times, as well as content and currency localization in How to Localize Your API .
One area that I did not get into was how timezones can play a role when a specific time and location are in play.
Simply put: you cannot tell what the UTC offset of a given date+time will be, unless you know precisely where and when it will be. As such, this becomes the defining line for when we should supply timezones.