Methodology
Current time and UTC offsets
Every live clock uses the JavaScript Intl.DateTimeFormat API against an IANA time zone identifier such as Europe/London or America/New_York. The IANA database is maintained as a public open-source project and is built into every modern browser and server runtime, so offsets and daylight saving status are correct for any date.
UK clock change dates
UK summer time starts at 01:00 UTC on the last Sunday of March and ends at 01:00 UTC on the last Sunday of October. Dates on this site are calculated from that rule, not typed in by hand, and the EU follows the same rule.
Time gaps between places
The gap between two places is the difference of their UTC offsets at a given moment. The table of when a gap changes is built by checking every day for the next two years, so it reflects both sides' daylight saving rules. Pages that show static tables are rebuilt around clock changes, while the live clocks and the current-gap statement update in your browser.
Sunrise and sunset
Sunrise and sunset times are calculated from each city's latitude and longitude using standard solar position equations, accurate to within a minute or two.
Population figures
City populations are rounded approximations of recent census figures, used for context only. They are not official statistics.