What it is for
Jet lag happens when your body clock is still running on the origin city's time while the wall clock says something else. This calculator shows the time difference between two cities on your travel date, the direction of travel, a rule-of-thumb estimate of how many days the body usually needs to adjust, and a simple pre-trip sleep shift.
How we calculate it
Each city uses its official IANA time zone (for example America/New_York), and your browser tells us the UTC offset in force at local noon on the chosen date. That means daylight saving time is handled correctly: London is UTC+0 in December and UTC+1 in July, while São Paulo is UTC−3 all year. The difference is destination offset − origin offset, and half-hour or 45-minute zones (Delhi is UTC+5:30, Kathmandu is UTC+5:45) work as expected.
The recovery rule of thumb
A widely repeated guideline in traveler health advice is that the body needs roughly one day per time zone crossed. Here the number of days is the hour difference rounded up. It is a pocket rule, not a medical model. Eastward flights (where you must advance your clock) are commonly reported to feel harder than westward ones, because the body's natural cycle runs a little longer than 24 hours, making it easier to delay than to advance. When the difference exceeds 12 hours we also show the effective shift going the shorter way around the 24-hour clock.
Shifting before you go
A common tactic is to move your sleep and wake times by about 1 hour per day toward the destination in the days before departure. Even a partial shift tends to help. This is a general suggestion, not a prescription.
Worked example
São Paulo (UTC−3) to Lisbon in July (UTC+1): a difference of +4 h, eastward, about 4 days of adjustment by the rule of thumb. In December Lisbon is back on UTC+0, so the difference is +3 h. São Paulo to Delhi comes out at +8 h 30 min.
Limits
The tool only accounts for time zones. Flight length, sleep quality, light exposure, age and personal habits change the real experience a lot. If you have a medical condition or take medication, talk to a health professional before changing your sleep routine.
Frequently asked questions
What is jet lag?
It is the temporary mismatch between your body clock and local time after crossing several time zones. It often causes sleepiness at odd hours, tiredness and trouble concentrating.
Why is travelling east usually harder?
You have to fall asleep earlier than your body wants, and the natural body cycle is a little longer than 24 h, so delaying the clock is easier than advancing it. It is a tendency, not a rule for everyone.
Does the calculator account for daylight saving time?
Yes. We use your browser's time zone data for the chosen date, so DST changes in Europe, the US, Australia and elsewhere are included automatically.
How reliable is the days estimate?
It is a rule of thumb (about 1 day per zone), useful as an order of magnitude. Real recovery varies between people and is not a medical prediction.