Scheduling international meetings requires understanding UTC (Coordinated Universal Time), the global standard from which all time zones are offset. Time zones are expressed as UTC-X or UTC+X. For instance, Tokyo is UTC+9, and New York is UTC-5 (during standard time).
To find the time difference between two locations, you subtract their offsets. Between Tokyo (UTC+9) and New York (UTC-5): 9 - (-5) = 14 hours. When it is 9:00 AM in New York, it is 14 hours later, or 11:00 PM, in Tokyo.
The Daylight Saving Time trap
The biggest trap in international scheduling is Daylight Saving Time (DST). Not all countries observe DST, and those that do change their clocks on different dates. The US usually changes clocks in early March and November, while Europe changes in late March and late October.
During the weeks between the US and European clock changes, the standard time difference shifts. For example, London is normally 5 hours ahead of New York. But for a few weeks in March, because the US springs forward before Europe does, the gap narrows to 4 hours. Recurring calendar invites often break during these weeks.
Non-integer time zones
Most time zones are offset by full hours, but several countries use half-hour or 45-minute offsets. India operates on Indian Standard Time (IST), which is UTC+5:30. Nepal uses UTC+5:45.
This means converting between New York (UTC-5) and Mumbai (UTC+5:30) yields a 10.5-hour difference. A 9:00 AM meeting in New York is at 7:30 PM in Mumbai. These offset complexities are why manual time zone arithmetic often fails, and specialized software or converters are required for global teams.