Time Zone Trip Planner

Local times and UTC offsets

Each destination carries an IANA time zone name such as Asia/Tokyo. The planner asks your browser for the UTC offset of that zone on the date you arrive, so daylight saving transitions are handled automatically; a trip to London in January and the same trip in July produce different offsets. Nothing is sent to a server and no time zone table ships with the page.

Daylight hours

Sunrise and sunset come from the standard sunrise equation. For the latitude and longitude of the city the tool works out solar noon for that day, then the sun's declination, then the hour angle at which the sun sits 0.833° below the horizon (the usual allowance for atmospheric refraction and the sun's radius). Daylight length is the span between the two. Above the polar circles the equation has no solution for part of the year; the planner reports midnight sun or polar night instead of inventing a sunrise.

The body clock model

Your body clock does not jump when the plane lands; it drifts. The planner tracks it as an offset that starts aligned with home and then moves toward each destination while you are there:

• Flying east means the local clock is ahead and you must advance. Advancing is the harder direction, so the model uses about 1 hour per day.
• Flying west means the local clock is behind and you must delay. Delaying is easier, so the model uses about 1.5 hours per day.

Any shift left over when you board the next flight is carried into the following leg. This is what makes a multi-stop trip different from a single hop: arrive in Singapore still four hours out of phase and the flight to Sydney starts from that four-hour deficit, not from zero.

Jet lag adjustment windows

The anchor is your core temperature minimum, roughly two hours before your habitual wake time. Light landing after that point pushes your clock earlier; light landing before it pushes your clock later. The planner places that low point on the destination clock and marks the windows accordingly:

• Advancing (east): seek bright light in the four hours after the low point, avoid light in the four hours before it.
• Delaying (west): seek bright light in the evening before the low point, avoid light in the morning after it.

Getting this backwards is worse than doing nothing, because light in the wrong window drags your clock away from local time. When a window falls in darkness a light box or a well-lit room is the practical substitute; when an avoid window falls in daylight, sunglasses and indoor light are enough.

Reading the strips

Every leg draws two bars covering local midnight to local midnight. The upper bar is the day, with the lit band running from sunrise to sunset and the vertical rule at solar noon. The lower bar is the light plan for arrival day, showing the seek window, the avoid window, and the sleep window your body clock is still running on.

Limits worth knowing

• Shift rates are population averages; morning types tend to advance more easily than evening types, and adjustment slows with age.
• Sunrise and sunset are computed for a flat horizon at sea level, so mountains and tall cities shift real first light by minutes.
• For stays of two nights or fewer the model often reports that full adjustment is not worth attempting; staying on home time is usually the better play.
• The plan covers light and sleep timing only. It is not medical advice.