Date & Time in PHP

Work with dates and times in PHP using date(), time(), strtotime(), and the DateTime class for formatting, comparison, and manipulation.

Introduction to Date & Time

PHP offers two approaches to working with dates: the procedural functions (`date()`, `time()`, `strtotime()`) and the object-oriented `DateTime` / `DateTimeImmutable` classes. The OOP approach is preferred in modern PHP because it is more readable and less error-prone.

1. What is a Date in PHP?

PHP represents dates as Unix timestamps (seconds since 1 Jan 1970 UTC).

Creating Dates

The `DateTime` and `DateTimeImmutable` classes are the modern way to work with dates in PHP. `DateTimeImmutable` is preferred — it never modifies itself; every operation returns a new object.

1. Current Date & Time

`new DateTime()` with no arguments creates an object for the current moment.

2. Date from String

Parse a date string using `new DateTimeImmutable($string)` or `strtotime()`.

3. Date from Components — setDate() / mktime()

Build a date from individual year, month, day values.

4. Date from Custom Format — createFromFormat()

Parse a date string that uses a non-standard format.

Timestamps

A Unix timestamp is the number of seconds elapsed since 1 January 1970 00:00:00 UTC. PHP uses it as the base unit for all date calculations.

1. Getting the Current Timestamp

`time()` returns the current Unix timestamp in seconds.

2. DateTime to Timestamp

`->getTimestamp()` converts a DateTime object to a Unix timestamp.

Getting Date Components

Use format characters to extract individual date components, or the `getdate()` / `date_parse()` functions for arrays of components.

1. Extracting Components via format()

Use single-character format codes to get individual parts.

2. getdate() — Component Array

`getdate()` returns an associative array of all date components.

Formatting Dates

PHP's `date()` function and `DateTime::format()` use the same format characters to produce output in any style — ISO 8601, human-readable, locale-aware, or custom.

1. Common Format Patterns

Format a date into ISO, human-readable, or custom strings.

2. Locale-Aware Formatting — IntlDateFormatter

Format dates in the user's locale using the `intl` extension.

Comparing Dates

`DateTimeImmutable` objects support direct comparison with `<`, `>`, `==`, and `<=>` operators. You can also compare via timestamps for arithmetic precision.

1. Comparing Two Dates

Use `<` / `>` directly on DateTime objects, or compare timestamps.

Date Arithmetic

PHP's `DateInterval` and the `modify()` / `add()` / `sub()` methods on `DateTimeImmutable` make date math readable and precise. The `diff()` method calculates the exact difference between two dates.

1. Adding & Subtracting — modify() and add()/sub()

Use `modify()` with English strings or `DateInterval` for precise intervals.

2. Difference Between Two Dates — diff()

`diff()` returns a `DateInterval` describing the gap between two dates.

3. Procedural Date Arithmetic — strtotime()

Use `strtotime()` for quick timestamp-based date math.

Timezones & UTC

PHP supports named timezones via the `DateTimeZone` class. Always store dates in UTC on the backend and convert to the user's local timezone only for display.

1. Local Time vs UTC

Create dates in a specific timezone using `DateTimeZone`.

2. Converting Between Timezones

Use `setTimezone()` to convert an existing date to another timezone.

Date & Time Best Practices

Date and time bugs are among the hardest to track down. Following a few simple rules eliminates most of them.

1. Recommended Practices

Use DateTimeImmutable, store UTC, and set a default timezone.