UTC Offset Arithmetic Basics

UTC is the single reference point. Every city's local time equals UTC plus its offset. Beijing sits at UTC+8. New York standard time is UTC-5. London winter time is UTC+0. These offsets can be positive or negative. A positive offset means the city is ahead of UTC. A negative offset puts it behind.

To find the time gap between two cities, subtract one offset from the other. Take Beijing at UTC+8 and London at UTC+0 in winter. The difference is 8 hours. Now compare Beijing with New York standard time at UTC-5. The subtraction gives 13 hours. The same subtraction works for any pair of zones.

Keep the signs straight when subtracting. For example, Beijing minus New York is 8 minus (-5), which equals 13. Beijing minus London is 8 minus 0, which equals 8. The result tells you how many hours apart the two local clocks are.

Here are the key offset values to remember:

  • Beijing: UTC+8, all year
  • London: UTC+0 in winter, UTC+1 in summer
  • New York: UTC-5 in winter, UTC-4 in summer

These numbers change with daylight saving in London and New York. China does not observe daylight saving. So the Beijing offset stays fixed. For any meeting window, you will use the correct seasonal offset for each city.

Daylight Saving Drift Handling

UK daylight saving runs from the last Sunday in March to the last Sunday in October. US daylight saving runs from the second Sunday in March to the first Sunday in November. In the two or three weeks when only one side has switched, London and New York are 4 hours apart instead of 5.

Daylight saving changes the offset between cities. New York moves to UTC-4 in summer, London moves to UTC+1 in summer, and China stays at UTC+8 all year. These shifts alter the gap between cities by one hour in each direction.

For Beijing and London, the difference is 8 hours in winter and 7 hours in summer. For Beijing and New York, it is 13 hours in winter and 12 hours in summer. The narrower summer gap makes overlap easier, but only by a single hour.

Never assume a fixed offset for cities that observe daylight saving. Instead, check the specific date and use a reliable time zone database. The calculation is one subtraction:

  • Take the UTC offset for each city on that date.
  • Subtract the smaller offset from the larger one.
  • If the result is less than 9 hours, working hours overlap.

For example, Beijing (UTC+8) and London (UTC+0) in winter give 8 hours, so a 9-hour workday overlaps by 1 hour. In summer, London becomes UTC+1, giving 7 hours, which leaves 2 hours of overlap. For Beijing and New York, winter yields 13 hours, so no overlap; summer yields 12 hours, still no overlap.

Work-Hour Overlap Window Math

To find a meeting time that works for two cities, compare their UTC offsets. A typical work day runs 0900 to 1800 local time, which is 9 hours. Two zones overlap in working hours only when the gap between their offsets is smaller than 9 hours. If the gap is 8 hours, the overlap is 1 hour. If the gap is 7 hours, the overlap is 2 hours. If the gap is 12 or 13 hours, there is no overlap at all.

Overlap hours = the larger of (9 minus the offset gap) and zero. For example, Beijing is UTC+8 and London is UTC+0 in winter, so the gap is 8 hours. That leaves 1 hour of shared work time. In summer, London shifts to UTC+1, making the gap 7 hours, so the overlap grows to 2 hours. Beijing and New York standard time differ by 13 hours (UTC+8 versus UTC-5), which gives zero overlap. During New York daylight saving, the gap narrows to 12 hours, still zero overlap.

Use these rules to pick a slot:

  • Gap of 8 hours: overlap is 1 hour, running around 0900 in the western zone and 1700 in the eastern zone.
  • Gap of 7 hours: overlap is 2 hours, typically 0900 to 1100 in one zone and 1600 to 1800 in the other.
  • Gap of 6 hours: overlap is 3 hours, a comfortable window for a standard meeting.
  • Gap of 9 hours or more: the formula would give zero or a negative number, so treat it as no overlap. Do not force a single meeting.

When the gap exceeds 9 hours, rotate the meeting among three cities to spread the awkward slot, so no single zone takes every meeting. For instance, Beijing, London, and New York can take turns hosting the early or late session. This keeps the burden fair and avoids one city waking at 0500 every time or working until 2200.

Worked Example Across Three Zones

Take three cities: Beijing (UTC+8), London (UTC+0 in winter), and New York (UTC-5 in winter). The gap between Beijing and London is 8 hours, so their overlap inside a 0900 to 1800 workday is one hour: 1700 to 1800 Beijing time is 0900 to 1000 London time. Beijing and New York differ by 13 hours, which exceeds the 9-hour workday, so overlap is zero. London and New York differ by 5 hours, giving a 4-hour overlap from 1400 to 1800 London time, which is 0900 to 1300 New York time.

That 4-hour window is the only viable pair. Beijing cannot join it, because 1400 to 1800 London time corresponds to 2200 to 0200 Beijing time, entirely outside the 0900 to 1800 workday. A three-city meeting therefore needs a compromise. One practical option is to shift the window to early morning or late evening for one zone.

For example, if the meeting runs 0800 to 1000 London time, that is 1600 to 1800 Beijing time and 0300 to 0500 New York time. New York bears the early-morning burden. Alternatively, 1600 to 1800 Beijing time gives 0800 to 1000 London and 0300 to 0500 New York. The same pattern repeats. A rotation across meetings can spread the awkward slot, so no single city wakes at 0500 or works until 2200 every time.

  • Beijing to London gap: 8 hours, overlap 1 hour.
  • Beijing to New York gap: 13 hours, overlap 0.
  • London to New York gap: 5 hours, overlap 4 hours.
  • Only London to New York works, from 1400 to 1800 London time.

Recurring Meetings and Fair Rotation

Rotating the meeting time across weeks prevents the same city from bearing the night shift every round. Shift the meeting by one hour each week for three weeks, then repeat the cycle.

For a three-city rotation involving Beijing, London, and New York, use these weekly slots:

  • Week 1: 0900 London, 1700 Beijing, 0400 New York.
  • Week 2: 1000 London, 1800 Beijing, 0500 New York.
  • Week 3: 1100 London, 1900 Beijing, 0600 New York.

This pattern spreads the burden. In Week 1, New York starts at 0400, which is outside its 0900 to 1800 working window. In Week 2, New York begins at 0500, still outside. In Week 3, New York starts at 0600, also outside. Beijing drops out of its work hours by one hour in week 3. The rotation does not fix every issue, but the same zone carries the early start for three weeks, then the rotation moves on.

Use a calendar tool that supports time zone conversion and recurring event exceptions. Set the recurrence to weekly, then manually adjust the start time by one hour for the second and third instances. After three weeks, reset to the Week 1 time. That keeps one offset fixed: Beijing is UTC+8 year round, London is UTC+0 in winter and UTC+1 in summer, New York is UTC-5 in winter and UTC-4 in summer. The offsets change seasonally, so check the tool's automatic conversion for the correct local times.