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World Clock Calculator

Takes a reference date and time in one IANA time zone and lists the corresponding local date, time, weekday and UTC offset in each zone of a list, using the tz database rules built into ICU (daylight saving included).

When to use

You are scheduling across several cities and need the local time in each of them for one instant, or want to see the spread of UTC offsets.

Do not use when: You only convert between two zones (use time-zone-converter) or need an epoch value (use unix-timestamp).

Formula

utc = reference wall time − offset_reference; for each zone: local = utc + offset_zone(utc); day_offset = local_date − reference_date

Offsets come from the IANA tz database via Intl.DateTimeFormat. Non-existent reference times (clocks jumped forward) are moved forward by the gap; ambiguous ones take their first occurrence.

Inputs

Parameter	Type	Required	Description
`date`	date	default today	Calendar date of the reference time in the reference zone; defaults to the current UTC date.
`time`	string	default 12:00	Wall-clock time in the reference zone, 24-hour HH:MM.
`zone`	string	default UTC	IANA time zone in which date and time are given.
`zones`	string_list	yes	List of IANA time zones to display (comma-separated or JSON array), e.g. America/New_York, Europe/London, Asia/Tokyo.

Outputs

Output	Type	Unit	Description
`utc_datetime`	string		The reference instant in UTC (ISO 8601).
`reference_datetime`	string		The reference time with its UTC offset (ISO 8601).
`clocks`	list		One entry per zone: zone, local_date, local_time, weekday, utc_offset, abbreviation and day_offset (local date minus reference date, in days).
`offset_span_hours`	number	h	Largest minus smallest UTC offset among the reference zone and all listed zones.

Example

09:00 Adelaide on 2026-09-23 in New York, London, Tokyo: {"date":"2026-09-23","time":"09:00","zone":"Australia/Adelaide","zones":["America/New_York","Europe/London","Asia/Tokyo"]} → {"utc_datetime":"2026-09-22T23:30:00Z","clocks":[{"zone":"America/New_York","local_date":"2026-09-22","local_time":"19:30","utc_offset":"-04:00","day_offset":-1},{"zone":"Europe/London","local_date":"2026-09-23","local_time":"00:30","utc_offset":"+01:00","day_offset":0},{"zone":"Asia/Tokyo","local_date":"2026-09-23","local_time":"08:30","utc_offset":"+09:00","day_offset":0}],"offset_span_hours":13.5}

New Year 2026 00:00 UTC in Auckland, Kolkata, Los Angeles: {"date":"2026-01-01","time":"00:00","zone":"UTC","zones":["Pacific/Auckland","Asia/Kolkata","America/Los_Angeles"]} → {"clocks":[{"zone":"Pacific/Auckland","local_date":"2026-01-01","local_time":"13:00","utc_offset":"+13:00"},{"zone":"Asia/Kolkata","local_time":"05:30","utc_offset":"+05:30"},{"zone":"America/Los_Angeles","local_date":"2025-12-31","local_time":"16:00","utc_offset":"-08:00","day_offset":-1}],"offset_span_hours":21}

GET https://tttkmbb.com/api/v1/calculate/world-clock?date=2026-09-23&time=09%3A00&zone=Australia%2FAdelaide&zones=America%2FNew_York%2CEurope%2FLondon%2CAsia%2FTokyo

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/world-clock (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/world-clock · Markdown: https://tttkmbb.com/time/world-clock.md · JSON definition: https://tttkmbb.com/time/world-clock.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="world-clock"
OpenAPI operationId: calculate_world_clock
Freshness: daily. Authentication: none. Rate limit: fair use (see rate limits).

Sources

IANA Time Zone Database (tz) (standard)
List of tz database time zones (Wikipedia) (reference)

FAQ

Why does a city show a different date?

day_offset is the local calendar date minus the reference date: −1 means the instant falls on the previous day there, +1 on the next day (e.g. Auckland is ahead of Los Angeles by up to 21 hours).

Can I use city names instead of zone names?

No; use the IANA zone of the city (Paris → Europe/Paris, Mumbai → Asia/Kolkata, Sydney → Australia/Sydney). Invalid names return an INVALID_PARAMETER error.

Related calculators

Time Zone Converter — Two-zone conversion with day difference and abbreviations.
Unix Timestamp Converter — Epoch seconds for the same instant.