Convert a time between cities, with daylight saving handled automatically
Times update live once a second when the date/time above matches now.
A city's clock time is its distance from UTC, the reference time zone the whole world's clocks are defined against, plus whatever seasonal adjustment that place currently observes. That second part is what makes timezone arithmetic error-prone by hand: a fixed number of hours works for only part of the year in any place that shifts its clocks for daylight saving, and the shift happens on different calendar dates in the northern and southern hemispheres, so "just add nine hours" is only ever true some of the time.
Rather than hard-coding offsets, this converter reads the browser's own built-in time zone database — the same one used to correctly display dates throughout the operating system — for every city, so a converted time already accounts for whichever rule that specific location is under on the specific date chosen, including places observing daylight saving and places that never do. A date shown a shade darker marks a conversion that lands on the previous or next calendar day, which is easy to miss with a plain hour-and-minute comparison and easy to get wrong when scheduling something across a wide date-line gap.
Add as many cities as needed to the list — useful for lining up a meeting across several offices at once — and leaving the date and time set to "now" turns the whole list into a live world clock that keeps ticking. Everything is computed locally from the browser's own clock and time zone data; nothing is looked up over the network or sent anywhere.