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The strftime function is used to format dates and times, returning a string based on the specified format. Its basic syntax is as follows:
<span><span><span class="hljs-title function_ invoke__">strftime</span></span><span>(</span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-variable">$format</span></span><span>, </span><span><span class="hljs-keyword">int</span></span><span> </span><span><span class="hljs-variable">$timestamp</span></span><span> = </span><span><span class="hljs-title function_ invoke__">time</span></span><span>()) : </span><span><span class="hljs-keyword">string</span></span><span>
</span></span>Here, $format specifies the date and time format, and $timestamp is the Unix timestamp to be formatted (defaulting to the current time). For example, formatting the current time with strftime:
<span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">strftime</span></span><span>(</span><span><span class="hljs-string">"%Y-%m-%d %H:%M:%S"</span></span><span>);
</span></span>This will output the current date and time, such as 2025-06-16 12:30:45.
When using strftime to format dates, the time zone is a crucial factor. By default, strftime formats the time based on the server’s current time zone. However, if the program runs under the wrong time zone, time conversion errors may occur, causing the displayed time to differ from expectations.
strftime follows the server’s default time zone settings. This means if the server’s time zone configuration is incorrect, the formatted output will be based on the wrong time zone. You can check the current time zone with date_default_timezone_get():
<span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">date_default_timezone_get</span></span><span>();
</span></span>If it is incorrect, you can set the right time zone using date_default_timezone_set():
<span><span><span class="hljs-title function_ invoke__">date_default_timezone_set</span></span><span>(</span><span><span class="hljs-string">'Asia/Shanghai'</span></span><span>);
</span></span>This ensures that when calling strftime, the returned time is formatted based on the correct time zone.
Although strftime can format time, it doesn’t directly support time zone conversion. To convert time between different time zones, the best practice is to use the DateTime class: first create a DateTime object, adjust its time zone, and then use strftime to format the output.
<span><span><span class="hljs-variable">$datetime</span></span><span> = </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-title class_">DateTime</span></span><span>(</span><span><span class="hljs-string">'now'</span></span>, </span><span><span class="hljs-keyword">new</span></span> </span><span><span class="hljs-title class_">DateTimeZone</span></span><span>(</span><span><span class="hljs-string">'UTC'</span></span>));
</span><span><span class="hljs-variable">$datetime</span></span>-></span><span><span class="hljs-title function_ invoke__">setTimezone</span></span><span>(</span><span><span class="hljs-keyword">new</span></span> </span><span><span class="hljs-title class_">DateTimeZone</span></span><span>(</span><span><span class="hljs-string">'Asia/Shanghai'</span></span>));
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">strftime</span></span><span>(</span><span><span class="hljs-string">"%Y-%m-%d %H:%M:%S"</span></span>, </span><span><span class="hljs-variable">$datetime</span></span>-></span><span><span class="hljs-title function_ invoke__">getTimestamp</span></span>());
</span></span>In this example, we first create a DateTime object in the UTC time zone, then use the setTimezone method to convert it to the Shanghai time zone (Asia/Shanghai), and finally format it with strftime.
Different regions may adjust time zones according to Daylight Saving Time (DST), which affects strftime outputs. In such cases, handling time zones and DST with the DateTime class is more reliable than relying solely on strftime. For example, certain parts of the US and Europe shift their time offsets seasonally. DateTimeZone automatically accounts for DST changes, while strftime does not.
For example:
<span><span><span class="hljs-variable">$datetime</span></span> = </span><span><span class="hljs-keyword">new</span></span> </span><span><span class="hljs-title class_">DateTime</span></span>(</span><span><span class="hljs-string">'2025-06-16 12:00:00'</span></span>, </span><span><span class="hljs-keyword">new</span></span> </span><span><span class="hljs-title class_">DateTimeZone</span></span>(</span><span><span class="hljs-string">'Europe/London'</span></span>));
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">strftime</span></span><span>(</span><span><span class="hljs-string">"%Y-%m-%d %H:%M:%S"</span></span>, </span><span><span class="hljs-variable">$datetime</span></span>-></span><span><span class="hljs-title function_ invoke__">getTimestamp</span></span>());
</span></span>Here, Europe/London automatically adjusts offsets depending on whether DST is in effect, while strftime simply formats based on the server’s local time zone.
When using strftime, be mindful of how different format specifiers interact with time zones. Common specifiers include:
%Y: Year
%m: Month
%d: Day
%H: Hour (24-hour format)
%M: Minute
%S: Second
However, strftime itself does not provide direct support for formatting time zone information. To display time zone data, consider using the format method of the DateTime class or calling date_default_timezone_get().
It’s important to note that strftime formats timestamps, which are integers representing seconds since January 1, 1970. Timestamps themselves are time zone independent. However, how a timestamp is interpreted depends on the local time zone. strftime formats the output based on the server’s time zone.
Therefore, if you want consistent timestamps across different time zones, you must ensure the correct time zone is specified when generating the timestamp.
When formatting dates and times with strftime, handling time zone conversions is critical. Developers should keep the following points in mind:
Time Zone Settings: Ensure the server’s time zone is correctly configured, or explicitly set it in the code.
Time Zone Conversion: Since strftime cannot handle conversions, use the DateTime class to adjust time zones before formatting with strftime.
Daylight Saving Time: Time zone offsets may change during DST transitions, and DateTimeZone automatically handles these adjustments.
Format Specifiers: strftime specifiers don’t directly support time zone output; consider combining it with the DateTime class.
Understanding these key points helps developers avoid common time-related errors in cross-time-zone applications and improves system reliability and accuracy.
