PHP date/time code must distinguish instants, zones, offsets, local calendar values, date-only values, calendar periods, and elapsed durations. Immutable objects and an injected clock keep those decisions visible.
Reliable temporal code parses exact contracts with warning checks, defines DST and month-end policy, stores canonical instants plus needed zone identity, formats explicit offsets, and tests gaps, overlaps, leap boundaries, and precision.
<?php
$utc = new DateTimeZone('UTC');
$kolkata = new DateTimeZone('Asia/Kolkata');
$meeting = new DateTimeImmutable('2026-07-11 10:00:00', $utc);
echo $meeting->setTimezone($kolkata)->format('Y-m-d H:i T');2026-07-11 15:30 ISTUse DateInterval for explicit calendar changes. Adding one month is a calendar operation and can produce surprising end-of-month results, so test dates near month boundaries.
<?php
$started = new DateTimeImmutable('2026-07-11');
$deadline = $started->add(new DateInterval('P14D'));
echo $started->format('Y-m-d') . ' -> ' . $deadline->format('Y-m-d');2026-07-11 -> 2026-07-25Use createFromFormat() when input follows a known format. Check both the return value and getLastErrors(), because parsing can succeed with warnings such as an out-of-range day.
<?php
$input = '31/12/2026';
$date = DateTimeImmutable::createFromFormat('!d/m/Y', $input);
$errors = DateTimeImmutable::getLastErrors();
$isValid = $date !== false
&& ($errors === false || ($errors['warning_count'] === 0 && $errors['error_count'] === 0));
echo $isValid ? $date->format('Y-m-d') : 'Invalid date';2026-12-31DateTimeImmutable methods return a new object. DateInterval calendar operations such as one month are not the same as adding a fixed number of seconds, and daylight-saving transitions can make a local day shorter or longer than 24 hours. State whether a rule is elapsed time or calendar time.
Parse external dates with a known format and inspect parsing errors instead of allowing ambiguous input. For database exchange, use a documented UTC format; for display, format only at the presentation boundary. Obtain the user’s time zone as an IANA identifier rather than guessing from an offset.
Separate an instant on the global timeline from a local calendar representation. A timestamp identifies an instant, while a local date and time needs a time-zone rule set to identify an instant and may be ambiguous or nonexistent during transitions.
Use `DateTimeImmutable` for application values so adjustments return new objects instead of mutating shared state. Assign every returned value; ignoring it leaves the original unchanged.
Inject a clock or current-time provider into business code. Calling `now` throughout the system makes tests cross midnight, deadlines inconsistent within one operation, and simulations difficult.
Store instants in a canonical UTC representation and store the user or event zone separately when future local scheduling matters. A fixed offset does not contain future daylight-saving rules.
A date-only value such as a birthday is not midnight UTC. Model dates, local times, durations, and instants as different concepts even when PHP exposes them through related classes.
Use `createFromFormat` when an external value promises a fixed syntax. Reset unspecified fields with the appropriate format control so parsing a date does not silently borrow the current clock time.
The parser can normalize out-of-range fields instead of rejecting them. Inspect `getLastErrors()` for warnings and errors and round-trip the formatted value when exact calendar validity is required.
Natural-language relative parsing is useful for trusted operator tools but ambiguous for public contracts. APIs and persisted data should use a documented machine format and explicit zone or offset.
Reject null bytes, trailing data, missing fields, invalid zones, and unsupported precision according to the boundary contract. Do not silently fall back to the server default zone.
Parse Unix timestamps as numeric instants rather than local date strings. Validate range against application and platform requirements before converting.
Use IANA zone identifiers such as `Asia/Kolkata` or `Europe/London` when calendar rules matter. Abbreviations can be ambiguous, and numeric offsets describe only one moment rather than a region rule history.
Converting an instant with `setTimezone` changes its displayed local fields without changing the instant. Reinterpreting the same wall-clock fields in another zone is a different operation and can change the instant.
Daylight-saving transitions create local times that occur twice or do not occur. Define whether scheduling rejects, chooses an occurrence, or shifts such times and test the exact zones your product supports.
Time-zone rules are maintained data. Keep the runtime and zone database current and record enough context to reproduce earlier outputs after a rule update.
The server default timezone should be explicit at bootstrap but not serve as hidden application policy. Pass zones to parsing and formatting boundaries.
Calendar arithmetic and elapsed-duration arithmetic answer different questions. Adding one calendar day can cross a DST transition, while adding 86,400 elapsed seconds always describes a fixed duration.
`DateInterval` can represent calendar components whose exact elapsed length depends on the starting instant and zone. Do not convert months or years into fixed seconds for billing, expiry, or age calculations.
Month-end arithmetic may overflow into a following month according to PHP rules. Define product behavior such as clamp-to-last-day or reject and implement that policy explicitly.
`diff` describes a calendar-oriented interval between objects and includes direction information. Test how the application consumes days, components, and invert rather than treating one field as a universal duration.
For latency and timeout measurement, use a monotonic clock API where available. Wall clocks can jump under synchronization and manual changes.
Use stable standards such as RFC 3339 with an offset for API instants and include fractional precision only when the contract preserves it. PHP format characters are case-sensitive and differ between parsing and display contexts.
Formatting is presentation, not localization. Human month names, calendars, and locale-specific ordering need internationalization tools and a selected locale rather than hard-coded English patterns.
Database timestamp types differ in range, zone conversion, and fractional precision. Define whether a column stores an instant or local schedule and verify driver round trips at boundaries and DST changes.
Sort and compare normalized instants, not localized display strings. For date-only business fields, compare date values in the governing calendar and zone policy.
Include an offset or `Z` in machine output so a consumer never guesses the producer default. Preserve original zone identity separately when users need future recurring local times.
Freeze or inject current time and test just before, at, and after every deadline. Include midnight, month end, leap day, year change, negative timestamps where supported, and maximum storage precision.
For each supported zone, test a DST gap, overlap, and normal date. Assert both the instant and displayed local result so a test cannot pass through the wrong interpretation.
Round-trip every external format and reject parser warnings, trailing data, impossible dates, invalid zones, and omitted offsets. Include mixed-version producer and database fixtures during migrations.
Avoid sleeping in tests. Advance a fake clock and separately integration-test platform scheduling or timeout behavior under bounded real time.
Log canonical instants, selected zone identifiers, and correlation context without relying on localized strings. This makes cross-region incidents reconstructable.
Recurring schedules need a local time, region zone, recurrence rule, and explicit skipped or duplicated occurrence policy. Recompute future instants from current zone rules instead of adding fixed seconds to the previous run. Store completed runs separately so rule updates do not alter prior results.
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