Time zones and jet lag

UTC: the reference

Every time zone is defined by its offset from UTC (Coordinated Universal Time), the international reference that replaced GMT for technical use. Example: Paris is UTC+1 in winter, UTC+2 in summer (daylight saving).

Common offsets (standard time, from UTC)

The International Date Line

Crossing the Pacific Ocean near the 180th meridian changes the calendar day — not just the time: a Tokyo → Los Angeles flight can technically "arrive the day before" it took off, local time.

Managing jet lag

• Getting daylight exposure as soon as you arrive helps reset your internal clock.
• Travelling east (losing time) is generally harder to adjust to than travelling west.
• Shifting your sleep schedule a few days before departure softens the impact.

UTC vs. GMT: what's the difference?

GMT (Greenwich Mean Time) is historically a solar time, measured by astronomical observation; UTC is its modern version, based on atomic clocks, more precise and stable. In practice, the two match to the second for everyday use — UTC is simply today's international technical and legal reference.

Planning an international call: the method

1. First convert your own time to UTC (subtract your offset).
2. Add the offset for the other person's time zone.
3. Check that both zones aren't switching daylight saving the same week — an offset can shift by an hour within a few days.

Common time zone codes