English
English
When calculating sunrise and sunset time using PHP, date_sunrise() and date_sunset() are two very useful functions. However, some developers have found that calling date_sunset() multiple times in the same script sometimes results slightly differently. This seemingly strange phenomenon actually has several potential technical reasons behind it.
First, let’s understand the basic usage of this function. date_sunset() is used to calculate the time when the sun sets based on a given date, geographical coordinates, and other parameters. Its basic syntax is as follows:
$timestamp = strtotime("2025-04-26");
$latitude = 34.0522; // The latitude of Los Angeles
$longitude = -118.2437; // Longitude in Los Angeles
$sunset = date_sunset($timestamp, SUNFUNCS_RET_STRING, $latitude, $longitude, 90.0, -7);
echo "The sunset time is：$sunset";
If you do not pass in the $timestamp parameter, you call the following form:
$sunset = date_sunset(time(), SUNFUNCS_RET_STRING, $lat, $lng);
Then the timestamp of the date_sunset() may be different each time, because time() returns the current time (in seconds) each time it is called. Even if there is only one second difference, the calculation of the sun's setting time may be slightly biased.
The internal calculations of date_sunset() rely on astronomical formulas, which usually involve floating-point operations. Due to rounding errors in the floating point number itself, if the parameters are slightly different (such as slight latitude or height deviation) when called multiple times, different returns may result.
The underlying layer of PHP's date_sunset() function calls the system's sunpos() or similar astronomical algorithm library. The calculation process may rely on system settings (such as time zones or daylight saving time rules) or compile parameters. If these settings change during script execution, it may also cause inconsistencies in the results.
PHP uses the time zone settings in php.ini by default. If you dynamically modify the time zone in the script, for example:
date_default_timezone_set("UTC");
// Then it was changed to
date_default_timezone_set("Asia/Shanghai");
Then, if you call date_sunset() in two different time zones, the results will naturally be different. Make sure to set the time zone uniformly before the call is called.
date_sunset() supports multiple return types (strings, timestamps, floating point numbers, etc.). If you use different SUNFUNCS_RET_* constants when multiple calls, the results will also vary. For example:
// Return string
$sunset1 = date_sunset($timestamp, SUNFUNCS_RET_STRING, $lat, $lng);
// Return timestamp
$sunset2 = date_sunset($timestamp, SUNFUNCS_RET_TIMESTAMP, $lat, $lng);
Although the two results represent the same time, the formats are different, which makes people mistakenly think that they are different "results".
Make sure to pass in a fixed $timestamp and do not rely on time() .
Set a unified time zone: date_default_timezone_set('Asia/Shanghai');
Avoid using different return types for comparison in one script.
Check whether the script changes parameters such as latitude, longitude, and height.
If accuracy is critical, consider using a dedicated astronomical computing library, such as https://m66.net/astrocalc .
date_sunset() is a convenient but detail-sensitive function. When using it, special attention should be paid to the consistency of parameters and the stability of the script environment to avoid seemingly "error" results due to small deviations. Understanding the implementation logic behind it will help us use such real-world-linked functions more accurately.
