strtotime() may produce incorrect date timestamps in the WordPress environment due to site time zone settings, resulting in inconsistency with standard PHP behavior. This article will delve into this issue and provide an effective solution based on wp_date(). By encapsulating a custom function, it ensures that strtotime() can return the expected UTC timestamp in the WordPress environment, thereby avoiding time zone-related date and time processing errors and improving the accuracy of date and time operations.

Understanding Date Time Handling in WordPress

In a standard PHP environment, the strtotime() function usually parses a datetime string according to the server's default time zone. In the WordPress ecosystem, however, things get more complicated. WordPress has introduced its own date and time management mechanism, allowing users to configure a specific time zone for the site through the "Time Zone" option in "Settings" -> "General". This setting will affect WordPress core functions such as wp_date() and current_time(), but the impact on the native strtotime() function may be unexpected, causing it to produce UTC timestamps that are inconsistent with expectations when processing date and time strings without time zone information.

Examples of problem symptoms:

Assume that the WordPress site is configured with a time zone of GMT 5. When we execute in a standard PHP compiler:

 var_dump(strtotime("2021-11-16 00:00:00"));

Usually you will get a timestamp representing UTC time 2021-11-16 00:00:00, such as 1637020800.

However, executing the same code in a WordPress site:

 var_dump(strtotime("2021-11-16 00:00:00"));

The result may be 1637002800. This timestamp is 5 hours less than expected (1637020800 - 1637002800 = 18000 seconds, or 5 hours), precisely because when WordPress handles strtotime() internally, it treats its input string "2021-11-16 00:00:00" as a time in the site's time zone (GMT 5), and then converts it to a UTC timestamp. Therefore, "2021-11-16 00:00:00 GMT 5" is actually equivalent to "2021-11-15 19:00:00 UTC", resulting in an incorrect UTC timestamp.

Solution: Combine wp_date() for time zone correction

In order to solve the time zone offset problem of strtotime() in WordPress, we can use the wp_date() function to "calibrate" the date and time string so that it can get the correct UTC timestamp when parsed by strtotime(). A key feature of the wp_date() function is its ability to format the timestamp according to the WordPress site’s time zone settings.

The following is the encapsulated solution function:

 /**
 * Fixed the time zone offset problem of strtotime() in WordPress.
 *
 * This function ensures that the given datetime string is converted to a UTC timestamp,
 * Even if the WordPress site is configured with a non-UTC time zone.
 *
 * @param string $str The date and time string to be converted.
 * @return int The corresponding UTC timestamp.
 */
function wp_strtotime($str) {
    // Step 1: Initial strtotime() call.
    // In a WordPress environment, this will convert $str (which is treated as the site time zone time) to a UTC timestamp.
    // For example, "2021-11-16 00:00:00" (GMT 5) will be converted to a timestamp of 2021-11-15 19:00:00 UTC.
    $initial_timestamp = strtotime($str);

    // Step 2: Use wp_date() to convert the UTC timestamp obtained in the previous step,
    // Reformat into a datetime string according to the WordPress site time zone.
    // At this time, if the site time zone is GMT 5, then 2021-11-15 19:00:00 UTC will be formatted as // "2021-11-16 00:00:00" (this string is the representation of the site time zone).
    $formatted_datetime_str = wp_date('Ymd H:i:s', $initial_timestamp);

    // Step 3: Use strtotime() again to convert this "calibrated" datetime string into a timestamp.
    // At this point, strtotime() will usually treat it as UTC time (or the server's default time zone, which has no effect if the server is UTC),
    // So we get the correct UTC timestamp we expect.
    // For example, "2021-11-16 00:00:00" will be converted to a timestamp of 2021-11-16 00:00:00 UTC.
    return strtotime($formatted_datetime_str);
}

Detailed explanation of how the function works:

1. First time strtotime($str): When you call strtotime("2021-11-16 00:00:00") in the WordPress environment, WordPress will interpret it as "2021-11-16 00:00:00" under the site time zone (such as GMT 5) and convert it to the corresponding UTC timestamp (i.e. 2021-11-15 timestamp of 19:00:00 UTC). This timestamp is "incorrect" because it has the time zone offset subtracted.
2. wp_date('Ymd H:i:s', $initial_timestamp): This step is key. The wp_date() function receives a UTC timestamp (the "incorrect" timestamp from the previous step) and formats it according to the configured time zone of the WordPress site. This means that wp_date() will reformat the timestamp of 2021-11-15 19:00:00 UTC into the string "2021-11-16 00:00:00" according to the time zone rules of GMT 5. Now, we have a string that looks the same as the original input string, but it has been "baptized" by WordPress time zone rules.
3. The second strtotime($formatted_datetime_str): At this time, we pass the string "2021-11-16 00:00:00" to strtotime() again. Since this string is "normalized" in the WordPress time zone environment through wp_date() and does not contain explicit time zone information, strtotime() will usually interpret it as UTC time in the WordPress environment at this time (or the server default time zone, if the server default time zone is UTC, the result is a UTC timestamp), thus returning the timestamp of 2021-11-16 00:00:00 UTC that we expect.

With this clever two-step conversion, we successfully offset the time zone offset imposed by WordPress's first call to strtotime() and end up with the correct UTC timestamp.

Usage example

Now, you can use this custom wp_strtotime function in your WordPress code:

 $date_string = "2021-11-16 00:00:00";
$correct_timestamp = wp_strtotime($date_string);

echo "Original date string: " . $date_string . "\n";
echo "Corrected UTC timestamp: " . $correct_timestamp . "\n";
echo "UTC date and time: " . gmdate('Ymd H:i:s', $correct_timestamp) . "\n";

// Verify that the behavior is consistent with standard PHP echo "Standard PHP strtotime result: " . strtotime($date_string) . "\n";

Run the above code, and you will see that the timestamp returned by wp_strtotime() is consistent with the result of strtotime() in the standard PHP environment, which is the correct timestamp representing "2021-11-16 00:00:00 UTC".

Notes and Summary

• Time zone awareness: In WordPress development, you must maintain a high degree of time zone awareness when dealing with dates and times. Confusing server time zone, PHP default time zone, and WordPress site time zone is a major cause of date and time errors.
• wp_date() and date_i18n(): wp_date() is the recommended date formatting function in WordPress 5.3. It replaces date_i18n() in the old version and provides more powerful time zone processing capabilities.
• The format of the input string: The wp_strtotime() function is suitable for parsing date and time strings without explicit time zone information. If your input string itself contains time zone information (e.g. "2021-11-16 00:00:00 GMT 8"), then the behavior of strtotime() will be more explicit and this correction function may not be needed.
• Code location: You can add the wp_strtotime function to your theme’s functions.php file or a custom plugin to make it available throughout your WordPress environment.

By adopting helper functions like wp_strtotime, developers can ensure that they always get the expected UTC timestamp when processing date and time strings obtained from user input or databases, thereby avoiding date calculation and display errors caused by time zone issues and improving the robustness of WordPress applications.

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