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3.4 Dates and TimesThere are several PHP library functions that work with dates and times. Most either generate a Unix timestamp or format a Unix timestamp in a human-readable form. Validation using dates, and working with the flexible PEAR Date package, is discussed in Chapter 9. In this section, we introduce timestamps and PHP library functions that work with Dates and Times. 3.4.1 Generating a TimestampA Unix timestamp consists of the number of seconds since the arbitrarily chosen time 1 January 1970 00:00:00 Greenwich Mean Time. Most systems represent a timestamp using a signed 32-bit integer, allowing a range of dates from December 13, 1901 through January 19, 2038. While timestamps are convenient to work with, care must be taken when manipulating timestamps to avoid integer overflow errors. While PHP automatically converts integers that overflow to floats, these values aren't valid timestamps.
3.4.1.1 Current timePHP provides several functions that generate a Unix timestamp. The simplest:
returns the timestamp for the current date and time, as shown in this fragment: // prints the current timestamp: e.g., 1064699133 print time( ); 3.4.1.2 Creating timestamps with mktime( ) and gmmktime( )To create a timestamp for a past or future date in the range December 13, 1901 through January 19, 2038, the mktime( ) and gmmktime( ) functions are defined:
Both create a timestamp from the parameters supplied; the parameters supplied to gmmktime( ) represent a GMT date and time, while the parameters supplied to mktime( ) represent the local time. This example creates a timestamp for 9:30 A.M. on June 18, 1998: $aDate = mktime(9, 30, 0, 6, 18, 1998); Both functions correctly handle parameter values that you might consider out-of-range. For example, the following call passes 14 for the month value, 29 for the day, and 2004 for the year, creating a time stamp for 1 March 2005: // Creates a time stamp for 1 March 2005 $aDate = mktime(0, 0, 0, 14, 29, 2004); Setting the month to 14 and the year to 2004 overflows to February in 2005 and setting the day to 29 overflows to the first of March. This characteristic allows scripts to add a quantum of time without range checking. The following example shows how 30 days can be added to a date and time: $paymentPeriod = 30; // Days
// generates a timestamp for 26 June 2002 by
// adding 30 days to 27 May 2002
$paymentDue = mktime(0, 0, 0, 5, 27 + $paymentPeriod, 2002);
// A different approach adds the appropriate number
// of seconds to the timestamp for 27 May 2002
$paymentDue = mktime(0, 0, 0, 5, 27, 2002)
+ ($paymentPeriod * 24 * 3600);If the components of a date are outside the range of dates the function is defined for, -1 is returned. Both functions allow the supplied date to be interpreted as daylight savings time by setting the flag is_dst to 1. The order of the arguments to these functions is unusual and easily confused. While the mktime( ) and gmmktime( ) functions are similar to the Unix mktime( ) function, the arguments aren't in the same order. 3.4.1.3 String to timestampThe strtotime( ) function generates a timestamp by parsing a human-readable date and time (between December 13, 1901 and January 19, 2038) from the string time:
The function interprets several standard representations of a date, as shown here: // Absolute dates and times
$var = strtotime("25 December 2002");
$var = strtotime("14/5/1955");
$var = strtotime("Fri, 7 Sep 2001 10:28:07 -1000");
// The current time: equivalent to time( )
$var = strtotime("now");
// Relative to now
print strtotime("+1 day"); // tomorrow
print strtotime("-2 weeks"); // two weeks ago
print strtotime("+2 hours 2 seconds"); // in two hours and two secondsCare should be taken when using strtotime( ) with user-supplied dates. It's better to limit the use of strtotime( ) to cases when the string to be parsed is under the control of the script. For example, it's used here to check a minimum age using a relative date: // date of birth: timestamp for 16 August, 1982
$dob = mktime(0, 0, 0, 8, 16, 1982);
// Now check that the individual is over 18
if ($dob < strtotime("-18 years"))
print "Legal to drive in the state of Victoria"; // prints3.4.1.4 Subsecond timesA Unix timestamp represents a date and time accurate to the second, but many applications require times to be represented to the subsecond. PHP provides the function:
The microtime( ) function returns a string that contains both a Unix timestamp in seconds and a microsecond component. The returned string begins with the microsecond component, followed by the integer timestamp: // prints the time now in the format "microsec sec" // Example: 0.55512200 1064700291 print microtime( ); One common use of the function microtime( ) is to generate an integer seed for a random-number generator: // Generate a seed. $seed = (float)microtime( ) * 1000000; // prints (for example) 555206 print $seed; Because the microsecond component appears at the start of the string returned from microtime( ), the returned value can be converted to a float with the (float) cast operator. Multiplying the float result by 1,000,000 ensures that you create a suitably varying integer. The following fragment shows how you can use both the microsecond and second components to create a floating point representation of the time: $parts = explode(" ", microtime( ));
$f = (float)$parts[0] + (int)$parts[1];
// prints (for example) 1064700291.56
print $f;3.4.2 Formatting a DateWhile the Unix timestamp is useful in a program, it isn't a convenient display format. The date( ) and gmdate( ) functions return a human-readable formatted date and time:
The format of the returned string is determined by the format argument. Passing in the optional timestamp argument can format a predetermined date. Otherwise, both functions format the current time. The format string uses the formatting characters listed in Table 3-3 to display various components or characteristics of the timestamp. To include characters without having the function interpret them as formatting characters, escape them with a preceding backslash character. The following examples show various combinations: // Set up a timestamp for 08:15am 24 Aug 1974
$var = mktime(8, 15, 25, 8, 24, 1974);
// "24/08/1974"
print date('d/m/Y', $var);
// "08/24/74"
print date('m/d/y', $var);
// prints "Born on Saturday 24th of August"
print date('\B\o\r\n \o\n l jS \of F', $var);
PHP also provides the equivalent functions:
The format string uses the same % sequence formatting character sequences as the C library function strftime( ). For example: // Prints "24/08/1974 08:15:25"
print strftime("%d/%m/%Y %H:%M:%S", mktime(8, 15, 25, 8, 24, 1974));
// Prints "08/24/1974"
print strftime("%D", mktime(8, 15, 25, 8, 24, 1974));The result of some % sequences used by strftime( ) and gmstrftime( ) depends on the locale settings. The following sets the locale so the printed date used the Estonian language and conventions: setlocale (LC_TIME, 'es');
// prints "laupäev 24 august 1974"
print strftime ("%A %d %B %Y", mktime(8, 15, 25, 8, 24, 1974));Formatting sequences supported by strftime( ) and gmstrftime( ) are shown in Table 3-4. Some of these % sequences are not supported under Windows as noted.
3.4.3 Validating a DateThe function checkdate( ) returns true if a given month, day, and year form a valid Gregorian date:
This function isn't based on a timestamp and so can accept any dates in the years 1 to 32767. It automatically accounts for leap years. // Works for a wide range of dates $valid = checkdate(1, 1, 1066); // true $valid = checkdate(1, 1, 2929); // true // Correctly identify bad dates $valid = checkdate(13, 1, 1996); // false $valid = checkdate(4, 31, 2001); // false // Correctly handles leap years $valid = checkdate(2, 29, 1996); // true $valid = checkdate(2, 29, 2001); // false |
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