タイムゾーンを変換する方法

问题

あるタイムゾーンの時刻を、別のタイムゾーンの相当する時刻にする必要があります。

前提条件

• UTC とタイムゾーンオフセットの理解
• 夏時間（DST）に関する知識

ステップ

1. 1. Parse the source time with its timezone

   Identify the source timezone (e.g., America/New_York, Asia/Tokyo, UTC). Parse the date and time in that timezone. Do not assume the system's local timezone matches the source.

   入力

   2023-10-14 10:00 America/New_York

   输出

   parsed with timezone

2. 2. Convert to UTC first

   Convert the source time to UTC as an intermediate step. This avoids directly computing offsets between two timezones, which is error-prone due to DST rules that differ by timezone and date. UTC is the universal pivot.

   入力

   10:00 EDT → UTC

   输出

   14:00 UTC

3. 3. Convert from UTC to the target timezone

   Convert the UTC time to the target timezone, applying the target's UTC offset and DST rules for that specific date. Use the IANA timezone database (via a library) for accurate, up-to-date offsets and DST transitions.

   入力

   14:00 UTC → Asia/Tokyo

   输出

   23:00 JST

期待される結果

夏時間とオフセットの違いを考慮した、目的のタイムゾーンでの正しい現地時刻。

よくあるエラー

Using fixed offsets instead of timezone names

Fixed offsets (like UTC-5) do not account for DST. Always use IANA timezone names (America/New_York) with a timezone-aware library.

System timezone assumed as source

Explicitly specify the source timezone. The system's local timezone may differ, especially on servers.

トラブルシューティング指引

• If the result is off by one hour, check for a DST transition on that date.
• Use the IANA timezone database, which is updated several times per year for rule changes.

セキュリティ上の制限

• Timezone conversion is a pure calculation with no security implications.