Date Calculators
Estimate how many days your body clock needs to adjust after a flight, from the time difference between your home city and your destination on your arrival date, with a simple plan for shifting your bedtime before you leave.
Choose your home city, your destination and the day you arrive, and the calculator works out how far the clock moves for your body, including daylight saving time in both places on that date. It then applies the rule of thumb most often quoted in sleep-medicine guidance: the body clock catches up by about one hour a day after flying east and about one and a half hours a day after flying west, because a day that is longer than 24 hours is easier to live with than a shorter one. A shift of more than 12 hours is counted the shorter way round the clock, so New York to Tokyo is treated as 11 hours west rather than 13 hours east. You also get a gentle head start: moving your bedtime one hour a day towards the destination for up to three days before you leave, and how many days of adjustment are left after that. Add the length of your stay to see whether you are likely to adjust before you return. The figures are averages for healthy adults, not medical advice.
Where:
Flying from New York (EDT, UTC−4) to Paris (CEST, UTC+2), arriving on 2 June 2025.
Clock shift, east because it is positive
Days to adjust at 1 hour a day
Head start, then the days left after landing
Result
Expect about 6 days to adjust to Paris time. Going to bed and getting up 1 hour earlier on each of the 3 days before departure cuts that to about 3 days after landing.
A common rule of thumb is about one day for each hour of time difference when you fly east, and about one day for every hour and a half when you fly west. New York to Paris in June is a 6-hour shift east, so about 6 days; Paris to New York is 6 hours west, so about 4 days. Individuals vary, and age, sleep before the flight and daylight at the destination all play a part.
Left to itself, the human body clock tends to run slightly longer than 24 hours, so it finds it easier to stay up later (a delay, as after flying west) than to fall asleep earlier (an advance, as after flying east). That is why the calculator allows 1.5 hours of adjustment a day westward but only 1 hour eastward.
Tokyo is 13 hours ahead of New York in June, but the body clock can just as well adjust by going 11 hours the other way, and a delay is easier than an advance. The calculator always takes the shorter way round the 24-hour clock, so the shift is −11 hours, west, and the estimate is about 8 days rather than 13. An exact 12-hour shift is also treated as westward.
Moving your sleep an hour a day towards the destination's time before you travel reduces the shift you have to make after you land. For New York to Paris, going to bed and getting up an hour earlier on each of the 3 days before departure leaves a 3-hour shift, about 3 days to adjust instead of 6. The calculator caps the head start at 3 days and never shifts more than the full time difference.
If your stay is shorter than the time needed to adjust, the calculator says so. On a 4-day trip from New York to Paris you would still be adjusting when you leave, and the return flight would set you back again; on trips this short some travellers prefer to keep close to home sleeping hours where they can.
Yes. London to Delhi in January is a shift of 5h 30m east, which rounds up to 6 days; with 3 days of head start the remaining 2h 30m rounds up to 3 days. Estimates are always rounded up to whole days.
Pick your home city, your destination and your arrival date to estimate how long jet lag lasts and how to get a head start.