About the Time Zone Converter
Converting time between zones looks like simple subtraction and is almost never that. The gap between two cities is not fixed: it depends on the date, because each country switches to and from daylight saving on its own schedule, and a handful of zones sit on half-hour or quarter-hour offsets. A 12:00 call between New York and London is a five-hour difference in January and a five-hour difference in July, but only because the two countries change their clocks within a week of each other — shift the date to late March and the answer changes.
That is why a fixed 'hours apart' table is unreliable and why this converter asks for a date. It reads the real offset for both zones on the exact day you specify, from the time zone database your own browser ships with, so the daylight saving transition is handled without you thinking about it.