Time Zone Calculator: Convert Times Across the World

This time zone calculator converts a single moment in time across multiple cities simultaneously. Add any city using the search bar and each one gets a card showing the local time, date, UTC offset relative to your location, and a day/night indicator. Change the time or date on any card and every other card updates instantly, all cards share one UTC timestamp displayed in each zone's local time. Green highlighting marks 9 AM–5 PM business hours so you can quickly find overlapping windows for meetings across time zones. The tool uses IANA time zone identifiers (e.g., America/New_York, Europe/London, Asia/Tokyo), which include daylight saving rules, so offsets switch automatically between standard and daylight time. Sun and moon icons show at a glance whether it is daytime or nighttime in each location. All processing runs locally in your browser, no data is transmitted.

Every local time can be expressed as a UTC (Coordinated Universal Time) timestamp. For a zone with offset $O$ hours from UTC, the relationship is $\text{UTC} = \text{Local} - O$: Tokyo with $O = +9$ at 10:00 local gives UTC = 10 − 9 = 01:00; New York in winter with $O = -5$ at 10:00 local gives UTC = 10 − (−5) = 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.

Click the search bar and type a city name to add it as a new time card, your local zone is shown by default. Each card displays the current local time, date, and how many hours ahead or behind it is compared to your computer's clock (e.g., "+5h local" or "−3h local"). Change the time or date on any card and all others update instantly to reflect the same moment. Green highlighting on each card marks 9 AM–5 PM local business hours, making it easy to spot when colleagues in different cities are available for meetings. Sun and moon icons indicate daytime (6 AM–6 PM) versus nighttime, helping you avoid scheduling calls at 2 AM for someone. Use the date picker to plan for future or past dates, daylight saving time shifts are handled automatically through IANA time zone rules. Click the trash icon to remove a card you no longer need; at least one card must remain.

We use IANA identifiers (America/New_York, Europe/London, Asia/Tokyo) instead of fixed offsets like "UTC-5." IANA includes daylight saving rules, when 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.