As mentioned, the preferred way to store dates in MySQL when working with PHP is to use Unix timestamps - large integers. However, this is not always the case - for example, if you use MySQL timestamps, you are able to make use of a wide variety of special functions within MySQL to help you work with dates, just as with PHP.
This advantage is minimal, however, because MySQL can convert from Unix timestamps as appropriate, so unless you are working with a database that will be read from a location not able to handle Unix timestamps - for example if you will be reading from it using PHP and Visual Basic for Windows, which cannot handle Unix timestamps - then you might want to use formatted dates.
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