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Travel tool

Jet Lag Calculator — Time Zones and Adaptation Days

Work out how many time zones you cross on your dates, which direction you fly, and how long adaptation takes, using the CDC's published rates.

Most jet lag advice says "adjust to the local time". This starts from the two things that actually decide how hard a trip is: how many hours your clock has to move, andwhich direction it moves. Both are computed from real UTC offsets on your dates, so a summer London–New York trip is not scored the same as a winter one.

Where these numbers come from

  • Adaptation rates — CDC Yellow Book 2026, Ch. 7.4 (Jet Lag Disorder): about 1 hour per day eastbound and 1.5 westbound, because the intrinsic circadian period is slightly longer than 24 hours.
  • Melatonin — the CDC gives 0.5–1mg as often sufficient for a circadian shift; the Cochrane review found doses above 5mg no more effective.
  • Offsets — every value is generated from the IANA time zone database by walking all 366 days of 2026 and recording exactly where the offset changes, verified2 October 2026. That is what caught the awkward cases: Delhi +5:30, Kathmandu +5:45, Morocco +1 except during Ramadan, and the countries that abolished daylight saving (Brazil 2019, Turkey 2016, Namibia 2017, Argentina 2009). It also matters at the edges — in 2026 Sydney changes on 5 April and 4 October, so a departure on 2 October is scored on the old offset.
  • What this tool deliberately does not do — it publishes no lux thresholds and no per-zone light schedules, because the CDC publishes neither. Published light-timing research anchors to the phase response curve (core body temperature minimum), not to clock hours, so a simple table here would be a guess wearing a citation. Use a proper circadian calculator for light timing.

A planning aid built on published rates, not medical advice. If jet lag regularly disrupts your life, talk to a clinician.

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Frequently asked questions

How many days does jet lag actually take?
The CDC Yellow Book puts average adaptation at one hour per time zone when flying east and 1.5 hours per zone when flying west, because your circadian period is naturally slightly longer than 24 hours. So a 5-hour eastward shift is about a 5-day adjustment and the same shift westward about 4. These are averages with real individual spread.
Is flying east or west worse?
On the physiology, east is harder: delaying your clock goes with the grain of a circadian period that is already slightly longer than 24 hours, while advancing fights it. Traveller reports are genuinely mixed and often describe the return flight home as the worse leg. This tool shows the measured direction for your dates rather than pretending the question is settled.
Why does my flight cross fewer time zones than the route suggests?
Because this tool compares real offsets on your travel date, not a flat map. London to New York is 5 hours in winter but only 4 in summer, because the two cities are on opposite sides of their own daylight saving. It also handles the half-hour and quarter-hour zones a naive converter rounds away, such as Delhi at +5:30 and Kathmandu at +5:45.
How accurate are these offsets?
Every offset is precomputed from the IANA time zone database for each month of the year, which is how the unusual cases were confirmed: Delhi at +5:30, Kathmandu at +5:45, Morocco at +1 except during Ramadan, and the countries that abolished daylight saving (Brazil in 2019, Turkey in 2016, Namibia in 2017, Argentina in 2009). The offsets are correct; daylight saving boundaries can change by government decision, so re-check before a trip years out.
Should I take melatonin, and how much?
The CDC says 0.5 to 1mg is often enough to shift the clock, taken close to your destination bedtime. The Cochrane review of ten randomised trials found doses above 5mg give no additional benefit, and that slow-release forms work less well than a short-lived peak. Mistimed melatonin can push you further out of alignment, so timing matters more than dose.