Timezone Handling

Also Known As

TL;DR

Explanation

Always store and compute in UTC; convert for display. PHP's DateTimeImmutable with DateTimeZone handles conversions. Database: use DATETIME with UTC application-level enforcement (MySQL), or TIMESTAMPTZ (PostgreSQL, which stores UTC and converts automatically). DST transitions create ambiguous times (1:30am exists twice) and gaps (2:30am may not exist). The IANA timezone database (America/New_York, not EST) handles DST rules correctly; three-letter abbreviations (EST, CET) are ambiguous.

Diagram

flowchart TD
    USER_INPUT[User enters: 2026-03-15 14:00] --> AMBIG{Which timezone?}
    AMBIG -->|assumed UTC| UTC_STORE[Store as UTC in DB<br/>2026-03-15T14:00:00Z]
    AMBIG -->|assumed local| WRONG[Wrong time stored!<br/>timezone lost forever]
    UTC_STORE --> DISPLAY[Display to user]
    DISPLAY --> CONVERT[Convert UTC to user timezone<br/>Europe/London = 14:00 BST]
    subgraph PHP_Best_Practice
        DI2[DateTimeImmutable with timezone]
        UTC2[Always store UTC]
        PREF[User timezone preference<br/>in profile settings]
        DI2 --> UTC2 --> PREF
    end
style WRONG fill:#f85149,color:#fff
style UTC_STORE fill:#238636,color:#fff
style UTC2 fill:#238636,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Storing local timestamps in the database — server TZ changes silently shift all stored times.
• Using date_default_timezone_set() to a non-UTC timezone for storage — always store UTC; convert only for display.
• Three-letter timezone abbreviations (EST, IST) — ambiguous; IST is India Standard, Israel Standard, and Irish Standard Time.
• Not using DateTimeImmutable — DateTime::modify() mutates the object; DateTimeImmutable always returns a new instance.

Code Examples

// Storing local time — breaks when server TZ changes:
date_default_timezone_set('America/New_York');
$created = date('Y-m-d H:i:s'); // Local time stored in DB
// Server moved to UTC: all existing times are now wrong

// Mutable DateTime — shared reference bug:
$start = new DateTime('2026-01-01');
$end = $start->modify('+30 days'); // $start is ALSO modified!

// UTC storage, local display:
date_default_timezone_set('UTC');
$created = (new DateTimeImmutable())->format('Y-m-d H:i:s'); // Stored as UTC

// Display in user's timezone:
$utc = new DateTimeImmutable('2026-01-01 12:00:00', new DateTimeZone('UTC'));
$local = $utc->setTimezone(new DateTimeZone('America/New_York'));
echo $local->format('Y-m-d H:i:s T'); // '2026-01-01 07:00:00 EST'

References

• ↗ https://www.php.net/manual/en/class.datetimeimmutable.php

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