Time calculator

Time Zone Difference Calculator

Compare current local times and offset differences between IANA time zones with automatic daylight saving time handling.

First time zone

Second time zone

Current offset difference

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How time zone offset differences are calculated

The calculator presents two dropdown menus, each containing 16 named IANA time zones such as America/New_York, Europe/London, and Asia/Tokyo. Select one zone in each dropdown. The calculator uses the browser's Intl API to determine the current UTC offset for each selected zone at the present moment. The difference is computed by subtracting the first zone's offset from the second zone's offset. A positive difference means the second zone is ahead; a negative difference means the second zone is behind.

The key strength of this approach is that it accounts for daylight saving time automatically. On any given date, the calculator looks up whether daylight saving is currently in effect in each zone and applies the correct offset. Because the calculation uses the current moment (the instant the calculator runs), the daylight saving status is always accurate for today. If you need to know the offset on a different date when daylight saving rules differ, you would need to calculate again on that date or use a different tool.

New York and London in summer versus winter

Offset difference = (UTC offset in zone B) − (UTC offset in zone A)

Positive difference = zone B is ahead of zone A

Negative difference = zone B is behind zone A

In June, New York observes Eastern Daylight Time (EDT), which is UTC−4. London observes British Summer Time (BST), which is UTC+1. The difference is (+1) − (−4) = 5 hours, so London is five hours ahead of New York during the summer.

In December, New York observes Eastern Standard Time (EST), which is UTC−5. London observes Greenwich Mean Time (GMT), which is UTC+0. The difference is (0) − (−5) = 5 hours, so London is still five hours ahead. Both zones shift on different dates, but the difference remains the same in this case. However, for a period in late March when the US has already shifted to daylight saving but the UK has not yet shifted, or in late October when the UK has shifted back but the US has not, the difference briefly changes to 4 hours.

Planning meetings across multiple time zones

A team with members in New York, London, and Tokyo must coordinate meeting times. New York to London is 5 hours (London ahead). Japan does not observe daylight saving, so London to Tokyo is 9 hours in winter and 8 hours during British Summer Time, and New York to Tokyo is 14 hours in winter and 13 in summer. If a January meeting is held at 9 AM in New York, it is 2 PM in London and 11 PM in Tokyo the same day. The calculator reveals these offsets instantly, letting you identify the least painful time slot for all participants.

Some zones use offsets that include fractional hours. India (Asia/Kolkata) uses UTC+5:30, adding 5.5 hours to UTC. If comparing Kolkata to New York (UTC−5 in winter), the difference is 5.5 − (−5) = 10.5 hours. Kolkata is 10 hours and 30 minutes ahead of New York in winter. This tool displays fractional differences correctly, so you can see the precise offset without manual calculation.

IANA zones, daylight saving transitions, and limitations

The calculator uses IANA time zone identifiers (e.g., America/New_York, Europe/London, Asia/Tokyo, Africa/Johannesburg). These represent regional rules, not fixed abbreviations like EST or GMT. A region's rules include daylight saving transitions, historical changes, and future adjustments.
Daylight saving time begins and ends on different dates in different regions. The US shifts in March and November, the EU shifts on different dates, and some regions do not observe daylight saving at all. The calculator reflects these differences automatically by checking the current date.
The provided list includes 16 major zones: America/New_York, America/Chicago, America/Denver, America/Los_Angeles, America/Toronto, America/Sao_Paulo, Europe/London, Europe/Paris, Africa/Johannesburg, Asia/Dubai, Asia/Kolkata, Asia/Singapore, Asia/Tokyo, Australia/Sydney, Pacific/Auckland, and UTC. It does not include every IANA identifier, only major economic and geographic centers.
The calculator shows the current date and time in each selected zone using your browser's Intl implementation. The browser must have up-to-date time zone data for accuracy. Older browsers or devices may have outdated rules.
Governments can and do change time zone rules. If a country adopts or abandons daylight saving time, the browser's time zone database must be updated. Until then, the calculator uses the old rule.
This tool compares the current instant in time, not a future meeting date. If daylight saving transitions occur between today and your meeting, the offset may differ on that day. Always verify on the actual meeting date or check the specific transition dates for the zones involved.

Frequently asked questions

Does this tool account for daylight saving time?

Yes. The calculator determines the current UTC offset for each zone on today's date, so daylight saving time is included if it is currently in effect. The offset changes automatically when daylight saving begins or ends in each zone.

Why can the time difference change during the year?

Because different regions start and end daylight saving time on different dates. When the US shifts to daylight saving in March but the UK has not yet shifted, the difference briefly becomes 4 hours instead of 5. The calculator always uses the current offset for today.

Are EST and EDT separate choices in the dropdown?

No. Choose 'America/New_York' and the calculator automatically applies the correct offset (EST in winter, EDT in summer) based on the current date.

What does the fractional hour offset mean?

Some regions use offsets like UTC+5:30 (e.g., India). A difference of 10.5 hours means 10 hours and 30 minutes. The calculator displays these fractional offsets exactly.

Can I calculate the time difference for a future meeting date?

This calculator shows the current offset. If your meeting is on a date when daylight saving transitions occur, the offset may differ. Always verify the offset on the actual meeting date.

Why do I need to choose an IANA zone name instead of a city or abbreviation?

IANA zones represent regional rules that include daylight saving and historical changes. Abbreviations like 'EST' are ambiguous (multiple regions use EST). City names work, but 'America/New_York' is the standard, unambiguous identifier.