If you've ever missed a meeting because you confused EST with EDT, or wondered why India is 5 hours and 30 minutes ahead instead of a round number, this guide is for you. Time zones seem simple until you actually need to use them.
How Time Zones Work
The Earth rotates 360° in 24 hours, so it's divided into 24 time zones of roughly 15° longitude each. The reference point is UTC (Coordinated Universal Time), which runs through Greenwich, London. Zones to the east are ahead (positive offsets), zones to the west are behind (negative offsets).
In practice, time zone boundaries follow political borders rather than neat longitude lines, which is why China — a country spanning 5 geographical time zones — uses a single time zone (UTC+8) for the entire country.
UTC vs. GMT
For everyday purposes, UTC and GMT are the same thing. GMT (Greenwich Mean Time) is the traditional name; UTC is the modern international standard. The only technical difference is that UTC is defined by atomic clocks while GMT is defined by the Earth's rotation. They differ by fractions of a second — irrelevant for scheduling meetings.
Major US Time Zones
| Zone | Standard | Daylight | Major Cities |
|---|---|---|---|
| Eastern | EST (UTC−5) | EDT (UTC−4) | New York, Miami, Atlanta |
| Central | CST (UTC−6) | CDT (UTC−5) | Chicago, Houston, Dallas |
| Mountain | MST (UTC−7) | MDT (UTC−6) | Denver, Phoenix (no DST) |
| Pacific | PST (UTC−8) | PDT (UTC−7) | LA, San Francisco, Seattle |
Daylight Saving Time (DST)
DST shifts clocks forward 1 hour in spring ("spring forward") and back in fall ("fall back"). In the US, this happens on the second Sunday of March and the first Sunday of November. During the weeks when one region has changed and another hasn't, the normal time difference between them changes — a 5-hour gap might temporarily become 4 or 6 hours.
Not everyone observes DST. Arizona (except Navajo Nation), Hawaii, and most of the world's countries near the equator don't change their clocks. Japan, China, India, and most of Africa and Southeast Asia also skip DST entirely.
Scheduling Across Time Zones
For remote teams, the key is finding overlap in business hours (roughly 9 AM – 6 PM local time for each participant). A few common scenarios:
US East Coast + Europe: Late morning EST (around 11 AM – 1 PM) = afternoon in Europe (4–6 PM CET). This is the sweet spot.
US + Asia-Pacific: The hardest pairing. Early morning US (7–8 AM EST) = evening Asia (8–9 PM JST). Someone usually has to compromise.
Europe + Asia-Pacific: European morning (8–10 AM CET) = afternoon/evening Asia (3–5 PM JST). Workable.