Dates and times are, unfortunately, not just collections of hours, minutes, seconds, months, days, and years. To be complete, they must also include a timezone. “Noon on October 20, 2016” is not the same instant in time in New York City as it is in London.
The PHP engine must be configured with a default timezone to use. The easiest way to do this is to set the date.timezone configuration parameter in your PHP configuration file. If you can’t adjust the file, call the date_default_timezone_set() function in your program before you do any date or time manipulation. In PHP 7, the engine defaults to the UTC timezone if you don’t specify your own default value.
There is a big list of possible timezone values that the PHP engine understands. Instead of using your local timezone, however, a convention that often makes software development easier is to set the timezone to UTC, the code for Coordinated Uni-versal Time. This is the time at zero degrees longitude and doesn’t adjust in the summer for a Daylight Saving Time setting. Although you have to do a little mental math to convert a UTC timestamp that appears, say, in a log file to your local time, using UTC makes it easier to work with time data that could be coming from multiple servers located in different timezones. It also avoids confusion during the switch to and from Daylight Saving Time because the apparent “clock time” doesn’t change.
Source: Sklar David (2016), Learning PHP: A Gentle Introduction to the Web’s Most Popular Language, O’Reilly Media; 1st edition.