Time Zone Calculator: World Clock & Meeting Planner
Convert times between cities. Free world clock for remote teams, meeting planners, and anyone asking "what time is it in Tokyo?" Runs in your browser, no sign-up.
The Math Behind Time Zone Conversion
Every local time can be expressed as a UTC (Coordinated Universal Time) timestamp. For a zone with offset hours from UTC, the relationship is : Tokyo with at 10:00 local gives UTC = 10 โ 9 = 01:00; New York in winter with at 10:00 local gives UTC = 10 โ (โ5) = 15:00. This calculator stores one UTC moment and displays it in each card's zone. Edit any card, we recompute UTC from that input, then update the rest.
Why IANA Time Zones Matter
We use IANA identifiers (America/New_York, Europe/London, Asia/Tokyo) instead of fixed offsets like "UTC-5." IANA includes daylight saving rulesโwhen New York switches from EST to EDT in March, the offset changes automatically. Same for London (GMT vs BST), Sydney, and 400+ other zones. Past and future dates get the right offset for that moment, so conversions stay accurate across seasons and years.
Use Cases: Meetings, Remote Work, Travel
- Meeting across time zones:Add your cities, pick a time that works. The green business-hours highlight shows when people are likely available. The "Xh local" tag tells you how far ahead or behind each place is from you.
- What time is it inโฆ?Classic world-clock question. Add the city, hit Now, and you see the current time there. Day/night icons tell you at a glance if you're about to wake someone up.
- Future or past planning:Use the date picker for any day. Deadlines, launch times, or "when does that webinar start for me?"โall work. DST is handled for the date you choose.
Time Zone Calculator FAQ
? How do I convert time between different cities?
Add each city with the search bar. Pick a time and date on any cardโthe others update instantly to show that same moment. The math: (offset in hours from UTC). Every card shares one UTC timestamp; we display it in each zone's local time.
? What does "+5h local" or "-3h local" mean?
That's how many hours ahead or behind each zone is compared to your computer's clock. +5h means when it's noon for you, it's 5 PM there. Handy when you're wondering "what time is it in London?" or planning a call.
? Why are some times shown in green?
Green marks 9 AMโ5 PM local timeโtypical business hours. Lets you spot when a remote colleague is likely at their desk so you can find overlapping hours for meetings.
? What are the sun and moon icons for?
Sun means daytime (6 AMโ6 PM) in that zone; moon means nighttime. Helps you avoid scheduling a call when it's 2 AM for the other person.
? Does this calculator handle daylight saving time?
Yes. We use IANA time zone IDs (e.g. America/New_York, Europe/London), which include DST rules. Offsets switch automaticallyโEST to EDT, GMT to BSTโso you don't have to guess.