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Time and Date 05_0672328887_CH03.qxd 6/8/06 2:38 PM Page 57 3 Time and Date APPLICATIONS THAT DEAL WITH TIME ARE forever present, especially in web-based applications: times of form submittals, user input such as date of birth, and the updating and removal of pages when out of date are just a few examples.Though potentially com- plicated, the date extensions in PHP are relatively simple and straightforward. This chapter deals mainly with the core date and time functions.These functions pro- vide all the basics for time manipulation: retrieving, formatting, and converting. One other extension, the Calendar functions, is devoted to conversion between different cal- endar systems, such as converting dates between the Gregorian and Jewish calendars. See the functions cal_to_jd() and cal_from_jd() in the “Quick Hits” section. A subject beyond the scope of this chapter is that of schedule or calendar manage- ment, such as meeting and event scheduling.An extension is available, called MCAL, which does provide such functionality. For more information, see http://php.net/mcal. To fully utilize the time functionality of PHP,you must understand how PHP meas- ures time. For PHP,time is measured as the number of seconds since January 1, 1970, the UNIX epoch. Hence any moment in time is stored as a simple integer.Time, when stored this way, is often referred to as a time stamp.As you will see, many functions either return a time stamp or use a time stamp as a parameter.You must be careful however, when reading documentation or code comments. Depending on context, the term time stamp can also be used in its more generic sense: a time at which something occurs. For exam- ple, when referring to web server log files, the time at which an event is logged is referred to as a time stamp. Quick Hits . Get the time stamp for the current time: $timestamp = time(); Taking no parameters, this function returns the time stamp of the current time—that is, when time() is called. Full documentation: http://php.net/time 05_0672328887_CH03.qxd 6/8/06 2:38 PM Page 58 58 Chapter 3 Time and Date . Get the current time stamp to the microsecond resolution: $result = microtime($format); Returns the UNIX time stamp but resolved to microseconds in accuracy. If $format is false, the default, the time is returned as the string “seconds microseconds” with the seconds portion the same as that returned by time(), and microseconds as a fraction of a second. If $format is true, the time is returned as a floating point number. Full documentation: http://php.net/microtime . Create a time stamp for a specified date and time: $timestamp = mktime($hour, $minute, $second, $month, $day, $year); Returns a time stamp that corresponds to the specified time and date. Full documentation: http://php.net/mktime . Format the specified time: $string = date($format, $timestamp); Returns a string representing the given time stamp based on the specified format. If $time- stamp is not specified, the current time is used. Full documentation: http://php.net/date . Format a date and time, adjusting the result to be Greenwich Mean Time (GMT): $string = gmdate($format, $timestamp); This function is equivalent to date(). However, the time is adjusted to be relative to GMT repre- sentation of the given time. Full documentation: http://php.net/gmdate . Get various information about a specified time: $time_array = getdate($timestamp); Returns an array of various values based on the given time stamp. The keys to the array are sec- onds, minutes, hours, mday, wday, mon, year, yday, weekday, month. Full documentation: http://php.net/getdate . Get and set the time zone used by the time functions: $string = date_default_timezone_get(); date_default_timezone_set($string); This pair of functions gets or sets the timezone identifier. Full documentation: http://php.net/date_default_timezone_set and http://php.net/date_default_timezone_get . Parse any English string representation of time and produce a time stamp: $timestamp = strtotime($english_time); This function can take nearly any string containing a textual description of a date and time, in English, and produce the corresponding time stamp. Full documentation: http://php.net/strtotime 05_0672328887_CH03.qxd 6/8/06 2:38 PM Page 59 3.1 Calculating the Difference Between Two Dates 59 . Validate a date against the Gregorian calendar: $isvalid = checkdate($month, $day, $year); Returns true if the specified date can be found on the Gregorian calendar. Useful to make sure that user input or calculated time is valid. Full documentation: http://php.net/checkdate . Find sunrise and sunset times for a given position: $time_rise = date_sunrise($day_ts, SUNFUNCS_RET_TIMESTAMP, $latitude, $lon- gitude); $time_set = date_sunset($day_ts, SUNFUNCS_RET_TIMESTAMP, $latitude, $longi- tude); This pair of functions returns the time of sunrise and sunset at the specified location. The con- stant, SUNFUNCS_RET_TIMESTAMP, tells the function to return the time as a time stamp. Full documentation: http://php.net/date_sunrise and http://php.net/date_sunset . Convert a date between different calendars and Julian Day Count: $jday_count = cal_to_jd($calendar, $month, $day, $year); $date_array = cal_from_jd($jday_count, $calendar); This pair of functions converts a date, in a specified calendar system, to and from a Julian Day Count. These are useful for converting dates from one calendar to another. The supported calendar systems are CAL_GREGORIAN, CAL_JULIAN, CAL_JEWISH, and CAL_FRENCH. Full documentation: http://php.net/cal_to_jd and http://php.net/cal_from_jd 3.1 Calculating the Difference Between Two Dates Finding the number of weeks, days, or even seconds between two dates is something that programs regularly need to do. Often a database will have powerful built-in procedures for comparing dates and doing date arithmetic. In PHP,though, you need to handle this yourself. As discussed in the introduction, PHP deals with dates in UNIX epoch format.This means that all dates are stored as the number of seconds since midnight, January 1, 1970. This makes it easy if you just need to know the number of seconds that have passed between two points in time.All you need to do is subtract the two values. However, doing the more common “days between” type arithmetic takes some extra care. While doing date math, you need to keep leap years, months with different numbers of days, months with a fifth week, years with a 53rd week, daylight savings time, and all other such variations in mind. Otherwise, you may find yourself with an algorithm that only works most of the time. Let’s begin with a look at the common problem of finding the number of days between two dates. In Listing 3.1.1, note that we will use the powerful function strtotime() to make the PHP date stamps for ourselves.This function takes many different forms of text representations of dates and converts them into a time stamp for you. 05_0672328887_CH03.qxd 6/8/06 2:38 PM Page 60 60 Chapter 3 Time and Date Note Since PHP 5.1 you must make a call to date_default_timezone_set() to specify what time zone you want calculations to take place in before calling any date functions. Otherwise, STRICT mode errors are generated as it tries to guess the appropriate time zone. More information about time zones is presented in section 3.4, “Handling Time Zones,” later in this chapter. If your code needs to run on both PHP 5.0 and PHP 5.1 systems, you will have a little bit of a problem because these functions don’t exist in PHP 5.0. Therefore, you can use the function function_exists() to check whether date_default_ timezone_set() exists, and if so, call it. Alternatively, the default time zone can be set in your PHP configuration file in the PHP 5.1 server and then you don’t need to worry about it. Listing 3.1.1 Number of Days Between Two Dates <?php // Set the default timezone to US/Eastern date_default_timezone_set('US/Eastern'); // Will return the number of days between the two dates passed in function count_days($a, $b) { // First we need to break these dates into their constituent parts: $a_parts = getdate($a); $b_parts = getdate($b); // Now recreate these timestamps, based upon noon on each day // The specific time doesn't matter but it must be the same each day $a_new = mktime(12, 0, 0, $a_dt['mon'], $a_dt['mday'], $a_dt['year']); $b_new = mktime(12, 0, 0, $b_dt['mon'], $b_dt['mday'], $b_dt['year']); // Subtract these two numbers and divide by the number of seconds in a // day. Round the result since crossing over a daylight savings time // barrier will cause this time to be off by an hour or two. return round(abs($a_new - $b_new) / 86400); } // Prepare a few dates $date1 = strtotime('12/3/1973 8:13am'); $date2 = strtotime('1/15/1974 10:15pm'); $date3 = strtotime('2/14/2005 1:32pm'); // Calculate the differences, they should be 43 & 11353 echo "<p>There are ", count_days($date1, $date2), " days.</p>"; echo "<p>There are ", count_days($date2, $date3), " days.</p>"; ?> You might think that this would be a simple problem. Subtract the two numbers and divide by the number of seconds in a day.This is true and would work, if you were 05_0672328887_CH03.qxd 6/8/06 2:38 PM Page 61 3.2 Determining Last Day of a Given Month 61 interested in a pure mathematical definition of a day. However, think about having two dates, such as November 11 at 11:00 p.m. and November 12 at 1:00 a.m.To the way we think about dates, there is one day between these dates. However, if you just divided as discussed previously, you would end up with only a two-hour difference. The solution is to recalculate the dates you are given to a set time of day, say noon.
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