1 day equals 86400 seconds?

Date operations are often used when writing code. Most of the time, for convenience, it is possible to simply add and subtract a timestamp:

$time = mktime(0, 0, 0, 8, 28, 2015);
$one_hour_later = date('Y-m-d H:i:s', $time + 3600);

This is a simple code to get the timestamp one hour later, but if you zoom in to the day When using the above units, is this timestamp really what you want?

For example, if you have a timestamp of 0 o'clock on a certain day and want to know the timestamp of 0 o'clock 5 days later, simply add the time after 5*86400, right?

$time = mktime(0, 0, 0, 3, 8, 2015);
$five_day_later = date('Y-m-d H:i:s', $time + 5 * 86400);
echo $five_day_later; 

>> 2015-03-13 00:00:00

It seems there is no problem. But what about people in another place executing it?

// 时区设置为美国芝加哥
date_default_timezone_set('America/Chicago');
$time = mktime(0, 0, 0, 3, 8, 2015);
$five_day_later = date('Y-m-d H:i:s', $time + 5 * 86400);
echo $five_day_later; 

>> 2015-03-13 01:00:00

Why isn’t it 0 o’clock 5 days later?

Of course, this key problem lies in daylight saving time (the specific principle of daylight saving time will not be introduced, you can Baidu yourself).

Solution: Use the strtotime function instead of addition and subtraction

// 时区设置为美国芝加哥
date_default_timezone_set('America/Chicago');
$time = mktime(0, 0, 0, 3, 8, 2015);
$five_day_later = date('Y-m-d H:i:s', strtotime('+5 days', $time));
echo $five_day_later;

Although this problem does not exist in those time zones that do not have daylight saving time, but for those with daylight saving time The time zone that exists at the time does have an impact on your expected results.