How The Global Clock calculates time

This page explains what The Global Clock calculates, where the underlying rules come from, how frequently values refresh, and where uncertainty remains. The short version: local civil time comes from your browser's time-zone implementation; solar values are estimates calculated from date and coordinates.

Local time and UTC offsets

The clock starts with the current instant reported by the visitor's device. JavaScript's Intl.DateTimeFormat formats that instant in a named IANA time zone such as America/New_York or Asia/Kathmandu. The displayed offset is derived for that specific instant, so half-hour and 45-minute offsets are preserved.

Daylight saving time

We do not apply a hand-written “add one hour” rule. DST status and transitions are inferred from changes in the browser's offset for the named zone. City pages scan the surrounding year for offset changes and refine a detected transition to approximately the nearest minute. Comparison pages use the same zone-aware offsets to show when the difference between two cities changes.

Sunrise, sunset, and solar noon

Solar times are calculated in the browser from the city's stored latitude, longitude, and current date. The implementation uses the standard astronomical approximation with a sunrise/sunset zenith of 90.833°, which accounts approximately for atmospheric refraction and the apparent radius of the Sun. Solar noon and day length are derived from the calculated rise and set times.

These are planning estimates, not observational forecasts. Local terrain, elevation, buildings, atmospheric conditions, and the precision of a city's representative coordinates can change what a person actually sees.

Refresh and update schedule

• Clock seconds: refreshed every second while the page is open.
• Current date and UTC offset: recomputed from the current instant when the page loads and as the clock runs.
• Sunrise and sunset: calculated on page load and refreshed hourly.
• DST details: calculated on page load from the browser's available time-zone rules.
• City and editorial data: reviewed when corrections, source updates, or material rule changes are identified.
• Sitemap freshness: live clock movement is not treated as a content modification. Sitemaps currently omit lastmod rather than publishing synthetic freshness dates.

Browser and device handling

Your browser supplies the current instant, its local zone, and support for named time zones. If the device clock is wrong, the live clock can also be wrong. If the operating system or browser carries outdated time-zone data, a recent government rule change may not appear until that software is updated.

Known limitations

• City coordinates represent a point near the city, not every address in the metropolitan area.
• Historical time-zone results depend on the historical coverage shipped with the browser.
• Future civil-time rules can change with little notice and may differ until vendors distribute an update.
• Abbreviations are display labels and can be ambiguous; named IANA zones are the authoritative identifier used by the site.
• Polar-day and polar-night cases may not have a conventional sunrise or sunset.

Accuracy policy

We aim to reproduce the civil-time rules available in current browsers and to label calculated solar values honestly. We do not claim atomic-clock synchronization, navigation-grade solar precision, or legal authority. Material errors are investigated against primary sources and corrected according to our corrections policy.