Introduction
In programming, especially when dealing with web applications or systems that record event times, it’s common to encounter UNIX timestamps. A UNIX timestamp is a long integer representing the number of seconds since January 1, 1970 (midnight UTC/GMT). To make these timestamps human-readable and useful in various contexts, we often convert them into formatted date strings.
This tutorial explores different methods to convert UNIX timestamps into ISO 8601 compliant date-time strings using PHP. We’ll cover basic functions, advanced object-oriented approaches with the DateTime class, and important considerations like timezone settings.
Understanding UNIX Timestamps
A UNIX timestamp is a straightforward way of representing time that avoids issues related to time zones and daylight saving changes. It’s particularly useful for storing dates in databases due to its simple numeric format.
For example:
- A UNIX timestamp like
1333699439represents the point in time: 2008-07-17T09:24:17Z.
Converting with PHP Functions
PHP provides several built-in functions that can convert timestamps into readable date strings. The two primary functions are date() and gmdate().
Using gmdate()
The function gmdate() is similar to date(), but it returns the time in Greenwich Mean Time (GMT), which is equivalent to UTC.
<?php
$timestamp = 1333699439;
echo gmdate("Y-m-d\TH:i:s\Z", $timestamp); // Outputs: 2008-07-17T09:24:17Z
?>
In this example, gmdate() formats the timestamp to an ISO 8601 date-time string.
Using date()
The date() function can be used with a specific format to achieve similar results. When converting to ISO 8601 format:
<?php
$timestamp = 1333699439;
echo date("Y-m-d\TH:i:s\Z", $timestamp); // Outputs: 2008-07-17T09:24:17Z
?>
Additionally, using date('c', $timestamp) gives an ISO 8601 formatted string with the local timezone offset:
<?php
echo date('c', $timestamp); // Example output: 2008-07-17T10:24:17+02:00
?>
Using PHP’s DateTime Class
From PHP 5.3 onwards, the DateTime class provides an object-oriented approach to handling dates and times. This method is more flexible and considered a modern best practice.
Basic Usage
To convert a timestamp into a formatted date string using DateTime, you can:
<?php
$timestamp = 1333699439;
$dateTime = DateTime::createFromFormat('U', $timestamp);
$formattedString = $dateTime->format('c'); // Outputs: 2008-07-17T09:24:17+00:00
echo $formattedString;
?>
Setting the Timezone
When dealing with timestamps, setting a default timezone ensures consistent and correct conversion results. Use date_default_timezone_set() before any date/time operations:
<?php
// Set the default timezone
date_default_timezone_set('Europe/Berlin');
$timestamp = 1307595105;
echo date('c', $timestamp); // Outputs: 2011-05-06T08:41:45+02:00
?>
Handling Milliseconds
Sometimes, timestamps are in milliseconds (e.g., JavaScript timestamps). In such cases, convert to seconds by dividing by 1000 before using PHP date functions:
<?php
$timestamp = 1486830234542;
echo date('Y-m-d H:i:s', $timestamp / 1000); // Outputs: 2017-02-10 08:33:54
?>
Conclusion
Converting UNIX timestamps to formatted date strings is a common task in PHP, and the language provides multiple ways to achieve this. Whether using simple functions like date() or adopting the object-oriented approach with DateTime, it’s crucial to consider timezone settings for accurate results.
By understanding these methods, you can effectively manage and display dates within your applications, ensuring they are both user-friendly and precise.