How Timezone Conversion Works

요약

Timezone conversion translates a local time in one timezone to the equivalent local time in another. The world is divided into timezone regions, each defined by a UTC offset that may change seasonally due to Daylight Saving Time (DST). The IANA Time Zone Database — maintained by ICANN and used by every major operating system and browser — is the authoritative source for these rules.

작동 방식

Every timezone is defined by its UTC offset — the number of hours and minutes ahead of or behind Coordinated Universal Time (UTC). To convert a time from one zone to another:

1. Determine the UTC offset of the source timezone on the given date
2. Determine the UTC offset of the target timezone on the given date
3. Calculate the difference and apply it to the source time

The date matters because many timezones observe Daylight Saving Time (DST), which shifts the UTC offset by one hour (typically) during summer months. For example, London is UTC+0 in winter (GMT) but UTC+1 in summer (BST).

Non-standard offsets

Most timezones use whole-hour offsets, but several do not:

공식

Business hour overlap

To find when business hours (typically 9:00–17:00) overlap across multiple timezones:

1. Convert each timezone’s business hours to UTC
2. Find the intersection of all UTC ranges
3. The number of overlapping UTC hours is the business hour overlap

계산 예시

입력값 설명

• From city — the source timezone city. Defaults to the user’s detected timezone.
• To city — the target timezone city.
• Time — the local time in the source timezone to convert.
• Additional cities — extra timezones for multi-zone comparison and meeting planning.

출력값 설명

• Converted time — the equivalent local time in the target timezone.
• Offset difference — how many hours/minutes ahead or behind the target is from the source.
• Day offset — whether the conversion crosses a date boundary (+1 day or −1 day).
• Business hour overlap — how many hours of standard working time (9–5) overlap between all selected timezones.
• 24-hour timeline — a visual strip showing which hours are business hours in each timezone and where they overlap.

가정 및 제한사항

• Business hours are assumed to be 9:00–17:00 local time. Many industries and cultures use different working hours.
• DST transitions — the calculator is DST-aware via the browser’s Intl API (backed by the IANA TZ Database), but the exact DST transition rules can change when governments update their policies.
• City list — the calculator includes 22 popular cities. Timezones not represented by a city in the list cannot currently be selected.
• Half-hour and 45-minute offsets — fully supported (India UTC+5:30, Nepal UTC+5:45, etc.).
• Historical dates — conversions use the current date; historical timezone rules are not modelled.

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