How to Convert Between Timezones

Problem

You have a time in one timezone and need the equivalent time in another timezone.

Prerequisites

• Understanding of UTC and timezone offsets
• Knowledge of DST

Steps

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.

   Input

   2023-10-14 10:00 America/New_York

   Output

   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.

   Input

   10:00 EDT → UTC

   Output

   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.

   Input

   14:00 UTC → Asia/Tokyo

   Output

   23:00 JST

Expected result

The correct local time in the target timezone, accounting for DST and offset differences.

Common errors

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.

Troubleshooting

• 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.

Security limits

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