Recipe 3.9. Parsing Dates and Times from Strings


3.9.1. Problem

You need to get a date or time in a string into a format you can use in calculations. For example, you want to convert date expressions such as "last Thursday" into an epoch timestamp.

3.9.2. Solution

The simplest way to parse a date or time string of arbitrary format is with strtotime( ), which turns a variety of human-readable date and time strings into epoch timestamps, as shown in Example 3-20.

Parsing strings with strtotime( )

<?php $a = strtotime('march 10'); // defaults to the current year $b = strtotime('last thursday'); $c = strtotime('now + 3 months'); 

3.9.3. Discussion

The grammar strtotime( ) uses is both complicated and comprehensive. It uses the GNU Date Input Formats specification, which is available at the following address: http://www.gnu.org/software/coreutils/manual/html_chapter/coreutils_27.html.

The function strtotime( ) understands words about the current time:

<?php $a = strtotime('now'); print strftime('%c',$a); $a = strtotime('today'); print strftime('%c',$a); ?> Mon Aug 12 20:35:10 2002 Mon Aug 12 20:35:10 2002

It understands different ways to identify a time and date:

<?php $a = strtotime('5/12/1994'); print strftime('%c',$a); $a = strtotime('12 may 1994'); print strftime('%c',$a); ?> Thu May 12 00:00:00 1994 Thu May 12 00:00:00 1994

It understands relative times and dates:

<?php $a = strtotime('last thursday');   // On August 12, 2002 print strftime('%c',$a); $a = strtotime('2001-07-12 2pm + 1 month'); print strftime('%c',$a); ?> Thu Aug  8 00:00:00 2002 Mon Aug 12 14:00:00 2002

It understands time zones. When the following is run from a computer in EDT, it prints out the same time:

<?php $a = strtotime('2002-07-12 2pm edt + 1 month'); print strftime('%c',$a); ?> Mon Aug 12 14:00:00 2002

However, when the following is run from a computer in EDT, it prints out the time in EDT when it is 2 P.M. in MDT (two hours before EDT):

<?php $a = strtotime('2002-07-12 2pm mdt + 1 month'); print strftime('%c',$a); ?> Mon Aug 12 16:00:00 2002

If the date and time you want to parse out of a string are in a format you know in advance, instead of calling strtotime( ), you can build a regular expression that grabs the different date and time parts you need. Example 3-21 shows how to parse "YYYY-MM-DD HH:MM:SS" dates, such as a MySQL DATETIME field.

Parsing a date with a regular expression

<?php $date = '1974-12-03 05:12:56'; preg_match('/(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})/',$date,$date_parts); ?>

This puts the year, month, day, hour, minute, and second into $date_parts[1] tHRough $date_parts[6]. (preg_match( ) puts the entire matched expression into $date_parts[0].)

You can use regular expressions to pull the date and time out of a larger string that might also contain other information (from user input, or a file you're reading), but if you're sure about the position of the date in the string you're parsing, you can use substr( ) to make it even faster, as shown in Example 3-22.

Parsing a date with substr( )

$date_parts[0] = substr($date,0,4); $date_parts[1] = substr($date,5,2); $date_parts[2] = substr($date,8,2); $date_parts[3] = substr($date,11,2); $date_parts[4] = substr($date,14,2); $date_parts[5] = substr($date,17,2); ?>

You can also use preg_split( ), as in Example 3-23.

Parsing a date with preg_split( )

<?php $ar = preg_split('/[- :]/',$date); var_dump($ar); ?>

Example 3-23 prints:

array(6) {   [0]=>   string(4) "1974"   [1]=>   string(2) "12"   [2]=>   string(2) "03"   [3]=>   string(2) "05"   [4]=>   string(2) "12"   [5]=>   string(2) "56" }

Be careful: PHP converts between numbers and strings without any prompting, but numbers beginning with a 0 are considered to be in octal (base 8). So 03 and 05 are 3 and 5, but 08 and 09 are not 8 and 9.

In PHP 5.1 and later, preg_match( ) is faster than strtotime( ) in parsing a date format such as "YYYY-MM-DD HH:MM:SS." In earlier versions of PHP, strtotime( ) is slightly faster. If you need the individual parts of the date string, preg_match( ) is more convenient, but strtotime( ) is obviously much more flexible.

3.9.4. See Also

Documentation on strtotime( ) at http://www.php.net/strtotime. The rules describing what strtotime( ) can parse are at http://www.gnu.org/software/coreutils/manual/html_chapter/coreutils_27.html.




PHP Cookbook, 2nd Edition
PHP Cookbook: Solutions and Examples for PHP Programmers
ISBN: 0596101015
EAN: 2147483647
Year: 2006
Pages: 445

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