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4.5 Working with Date and Time

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Being able to work easily with date and time information is one of the most basic skills every PHP programmer should get under their belt as soon as possible, so this is where we will be kicking off.

PHP represents time as the number of seconds that have passed since January 1st 1970 00:00:00 GMT, a date known as the start of the Unix epoch, hence this date format is known as "epoch time" or a "Unix timestamp". This might seem like a peculiar way to store dates, but it is actually remarkably good - internally you can store any date since 1970 as an integer, and convert to a human-readable string wherever necessary.

<< 4.4 Controlling script execution: exit(), eval(), and die()		4.5.1 Reading the current time: time() and microtime() >>
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A PHP User - 07 Sep 2008

you can also store dates prior to 1970 as negative numbers,
but PHP may not support calculations involving them, escpcially date from before the introduction of the gregorian calendar.

(if php uses the C time libraries you'll be limited to the dates supported there... 1970-2038 on many systems. this limit will be increased as systems/libraries migrate from int_32 for time_t, to float, or int_64)

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