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タイムゾーンコンバーター

Convert a date and time across multiple time zones simultaneously. Enter a source date, time, and time zone, then add as many target time zones as needed to see the converted times side by side. Supports 24-hour format toggle and all IANA time zones including DST-aware conversions.

使い方

Set your source date and time, select the source time zone from the searchable dropdown, then use Search & Add Timezone to add destination time zones. All conversions update instantly. Toggle 24-hour format to suit your preference.

特徴

Multi-zone comparison – add unlimited destination time zones
DST-aware – handles daylight saving time transitions correctly
Full IANA timezone database – all standard time zones with city names
Searchable zone selector – find zones by city, country, or UTC offset
24-hour toggle – switch between 12-hour and 24-hour display
Real-time updates – conversions update as you change any input

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 よくある質問

What is the difference between UTC, GMT, and a time zone?

UTC (Coordinated Universal Time) is the primary time standard that clocks and time are regulated by, maintained by atomic clocks. GMT (Greenwich Mean Time) is a time zone at 0° longitude that is equivalent to UTC in winter but is a geographic time zone, not a time standard. A time zone is a region that observes a uniform standard time, defined by its UTC offset and DST rules. Most programming and systems use UTC as the baseline and convert to local time for display.
How does daylight saving time affect time zone conversions?

DST shifts clocks forward (typically by 1 hour) during summer months to extend evening daylight. Not all regions observe DST, and those that do transition on different dates. When converting across DST boundaries, the UTC offset of a time zone changes — for example, Eastern Time is UTC−5 in winter (EST) and UTC−4 in summer (EDT). Always use IANA time zone IDs (e.g., America/New_York) rather than abbreviations (EST/EDT) for unambiguous DST-aware conversion.
Why do time zone abbreviations cause confusion?

Many abbreviations are shared by multiple time zones: IST is India Standard Time (UTC+5:30), Israel Standard Time (UTC+2), and Irish Standard Time (UTC+1). CST is Central Standard Time (UTC−6), China Standard Time (UTC+8), and Cuba Standard Time (UTC−5). Using IANA identifiers (e.g., Asia/Kolkata, America/Chicago) eliminates ambiguity entirely. Abbreviations should only be used for display, never for parsing or calculation.
What are the most common pitfalls in time zone handling for developers?

The most common pitfalls are: storing local times instead of UTC in databases, ignoring DST transitions when adding time intervals, using 3-letter abbreviations instead of IANA IDs, assuming all days have 24 hours (DST transitions create 23- and 25-hour days), and treating UTC offset as static (many zones change offset seasonally). Always store timestamps in UTC, convert to local time only at display time, and use a library with the IANA timezone database (Intl.DateTimeFormat, Moment-Timezone, date-fns-tz).