Instantly compare any two global time zones across the globe. See current local times, calculate the exact hour & minute difference, detect daylight saving time, and visualize time zones on an interactive band.
A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes. Time zones are defined by their offset from Coordinated Universal Time (UTC), the primary time standard by which the world regulates clocks and time. The concept of time zones was first proposed by Sir Sandford Fleming in the late 19th century and was formally adopted at the International Meridian Conference in 1884, which established the Greenwich Meridian as the prime meridian (0° longitude).
UTC offset = Local Time − UTC
For example, New York (EST) is UTC−5, while Tokyo (JST) is UTC+9. The difference is 14 hours.
Today, the world is divided into approximately 40 standard time zones, each typically spanning 15° of longitude (one hour of solar time). However, political and geographical boundaries often cause deviations, with some time zones using offsets of 30 or 45 minutes (e.g., India UTC+5:30, Nepal UTC+5:45). The International Date Line (IDL) roughly follows the 180° meridian and marks the boundary where the calendar date changes by one day when crossed eastward or westward.
Our calculator uses the IANA Time Zone Database (also known as the Olson database) to accurately determine the current UTC offset for each selected location. The IANA database is the global standard for time zone information and is maintained by a community of volunteers under the stewardship of the Internet Assigned Numbers Authority (IANA). It accounts for historical changes, daylight saving time rules, and political adjustments.
For each selected time zone, we retrieve the current date and time using the browser's built‑in Intl.DateTimeFormat API with the timeZone option. This gives us a precise local time string. The UTC offset is derived by comparing the local time to UTC, and the difference between the two zones is computed as:
Where a positive ΔT means Zone B is ahead of Zone A (eastward), and a negative ΔT means Zone B is behind (westward). The total difference is expressed in hours and minutes, with proper handling of half‑hour and quarter‑hour offsets.
Additionally, the calculator detects whether the two zones are on different calendar dates (i.e., whether the International Date Line is crossed). This is particularly important for scheduling across the Pacific Ocean, where a flight from Los Angeles to Auckland can result in a date change of +2 days.
Daylight Saving Time (DST) is the practice of advancing clocks during the warmer months so that darkness falls later each day according to the clock. The typical DST shift is +1 hour in spring (often called "spring forward") and −1 hour in autumn ("fall back"). DST is observed in many countries, but the start and end dates vary widely — for example, the United States begins DST on the second Sunday in March, while the European Union begins on the last Sunday in March.
Our calculator automatically detects whether DST is currently active in each selected time zone by querying the IANA database. The DST status is displayed alongside the time difference, helping you avoid the common confusion that arises when scheduling across DST transitions. It is important to note that not all regions observe DST — countries near the equator typically do not, as the length of daylight changes very little throughout the year.
A multinational technology company is planning a simultaneous global product launch at 10:00 AM UTC. The launch team is distributed across San Francisco (UTC−7 during PDT), London (UTC+1 during BST), Singapore (UTC+8), and Sydney (UTC+10 during AEST). Using our Time Zone Difference Calculator, the team can determine that the launch corresponds to:
This information is critical for planning press releases, social media campaigns, and customer support coverage. The calculator also reveals that San Francisco and London are on different dates for part of the year due to DST transitions, which the team must account for in their communications calendar.
The International Date Line (IDL) is an imaginary line on the surface of the Earth that runs from the North Pole to the South Pole and demarcates the boundary between one calendar day and the next. It roughly follows the 180° meridian but deviates in several places to avoid splitting countries or island groups. When you cross the IDL traveling eastward, you subtract one day; when traveling westward, you add one day.
Our calculator indicates whether your selected time zones are on opposite sides of the IDL, which is especially useful for travelers and logistics professionals. For example, a flight from New York (UTC−5) to Auckland (UTC+12) results in a time difference of 17 hours and a date difference of +1 day (or more during DST transitions).