A time zone converter shows what a given moment is in other parts of the world, allowing for daylight saving where it applies. Pick a date and time in one city, and see the exact same moment across every major time zone at once. Handles daylight saving automatically, using your device's own time zone data - not a lookup table that goes stale.
Time zone rules change more often than people expect - countries adjust their daylight saving dates, and some abolish or introduce it entirely. Rather than shipping our own list of rules that would slowly go stale, this uses your browser's built-in time zone database, the same one your phone's clock relies on. It updates itself when your device does, so this page never falls behind.
Usually 4am, since New York is five hours behind London - but the gap changes for a few weeks each year because the two regions switch daylight saving on different dates.
Yes, and it is the most common source of error. Countries change on different dates, and the southern hemisphere runs opposite to the northern.