Is anything sent to a server?

Nothing. Conversions use the browser's built-in Intl API, and the world map is drawn from land geometry bundled into the page itself — so not even the map triggers a request. Your times and zones never leave the tab.

Why does a city show +1d or −1d?

The same instant falls on a different calendar date there. Tokyo 09:00 Monday is Sunday evening in San Francisco — the badge is what stops you booking a meeting on the wrong day.

How is the day/night line drawn?

From the sun's subsolar point (declination and hour angle) at the selected instant. Every lit and unlit dot is the actual state of the planet at that moment, not an illustration.

Does the tool handle daylight saving time?

Yes, by construction. It formats with IANA timezone identifiers (like America/New_York) through Intl, so the browser's own timezone database supplies the correct offset for the exact instant — including the days a DST transition happens.

Timezone Converter

Overview

Scheduling a release, a meeting, or an incident review across regions means translating one moment into many local clocks. This tool shows a single instant in every zone you care about, together with each zone’s UTC offset, the difference from your own zone, and a badge when the calendar date differs.

The map is not decoration. The day/night boundary is computed from the sun’s real position at the selected instant, so “it’s the middle of their night” is something you see rather than something you work out.

How to use

Scrub the timeline — every clock and the day/night line move together, up to 24 hours either side of now.
Watch the +1d / −1d badges. That date boundary is the classic trap in meeting invites.
Click any row to copy its ISO 8601 timestamp, complete with the zone’s offset.
Use + add zone to add any IANA zone your browser knows, and now ⏺ to jump back to the present.

Examples

Booking a 09:00 Tokyo standup with San Francisco: scrub to 09:00 and read the −1d badge — for them it is the previous evening.
Planning a 16:00 UTC deploy: scrub until UTC reads 16:00 and check that nobody on the list is in the middle of their night.
Filing a timestamp in a ticket: click the row for the zone the incident was reported in and paste the ISO 8601 value.

Notes

Zones in the tool’s own table show a day/night dot and appear as a point on the map. Zones added beyond that table still convert exactly — they simply have no coordinates, so no position is guessed for them.

Terminator accuracy is about ±0.5°; atmospheric refraction and the sun’s angular radius are deliberately ignored. That is well inside the width of a dot on the map.