Section 11.2. Date and Time Functions

11.2. Date and Time Functions

PHP uses the standard Unix-style timestamp to work with dates. This is simply the number of seconds since January 1, 1970. You get the current timestamp using the time function, shown in Example 11-15.

Example 11-15. A simple echo of the timestamp

 <?php $timestamp= time(); echo $timestamp; ?> 

This results in Figure 11-13.

Figure 11-13. A Unix timestamp

 1134511981 

This is not exactly the most meaningful representation of the date and time. So instead, you can use the date function to translate the timestamp into a meaningful string. The date function takes a timestamp and a format string, as shown in Example 11-16.

Example 11-16. Making the date and time appear like we expect

 <?php $timestamp= time(); echo date("m/d/y G.i:s",$timestamp); ?> 

This code returns the screen shown in Figure 11-14.

Figure 11-14. An easy-to-read date and time from the date function

Dates and times can be displayed in a variety of formats; these will be discussed next.

11.2.1. Display Formats

Date and times are displayed in a variety of formats. In Example 11-16, we used a date format string of m/d/y G.i:s. Table 11-2 shows other possible components for those formats.

Table 11-2. Time-formatting values

Format	Meaning	Example value
a	am or pm	Am
A	AM or PM	AM
d	Day of the month	01
D	Day or the week	Sun
F	Month name	January
h	Hours in 12-hour format with leading zeros	04
H	Hours in 24-hour format with leading zeros	16
g	Hours in 12-hour format without leading zeros	4
G	Hours in 24-hour format without leading zeros	16
i	Minutes	35
j	Day of the month	2
l	Day of the week as a name	Sunday
L	Leap year (1 for yes, 0 for no)	1
m	Month of the year abbreviated to three characters	Jul
M	Month of the year	July
N	Month of the year as number without leading zeros	7
s	Seconds of the hour	58
S	Suffix for the day	th, nd, st, rd
R	Standardized date format	Thu, 15 Dec 2005 16:49:39-0600
U	Timestamp	1134512479
y	Two-digit year	25
Y	Four-digit year	2025
z	Day of year	234
Z	GMT offset in seconds (Greenwich Mean Time)	-21600 (-6*60*60)

11.2.2. Arithmetic

Adding or subtracting days and hours can be done by adding or subtracting seconds. While this may sound odd, it's not hard. To add two days to a timestamp, add 2*24*60*60 (2 days*24 hours*60 minutes*60 seconds) to the timestamp, as shown in Example 11-17.

Example 11-17. Adding two days to the date

 <?php $timestamp= time(); echo date("m/d/y G.i:s",$timestamp); $seconds=2*24*60*60; $timestamp+=$seconds; echo "<br>new dates   is:"; echo date("m/d/y G.i:s",$timestamp); ?> 

This outputs:

 12/13/05 16.28:32 new dates is:12/15/05 16.28:32 

Let's see what else you need to create dates with validation.

11.2.3. Validating Dates

When you receive a user-supplied date, it's good practice, as with any other user-supplied data, to check that it's valid. You can use the checkdate function, shown in Example 11-18, to validate a date. It takes three parametersthe month, day, and yearfor a date to validate. If the date is valid, it returns trUE; otherwise, it returns FALSE.

Example 11-18. Validating two dates

 <?php   echo("Validating: 4/31/2005<br>");   if (checkdate(4,31,2005)) {     echo('Date accepted.');   }   else {     echo ('Invalid date.');   }   echo("<br>");   echo("Validating: 5/31/2005<br>");   if (checkdate(5,31,2005)) {     echo('Date accepted.');   }   else {     echo ('Invalid date.');   } ?> 

As you can tell by our example in Figure 11-15, the 31 April 2005 date was invalid, yet 31 May 2005 was valid. This can happen because of a typo or a user just entering wrong information.

Figure 11-15. Only some months have 31 days

Once you know that you have the valid segments of a date, you can create a timestamp.

11.2.4. Using mktime to Create a Timestamp

It's fairly easy to get the current time and date using date, but if you're trying to create a date and all you have are the components of the date such as month, day, and year, you'll need to use the mktime function. The mktime function takes these parameters:

• Hour

• Minute

• Second

• Month

• Day of the month

• Year

You can omit some of the parameters when calling mktime, and they'll be filled in from the current time. mktime is a timestamp, which is an integer containing the number of seconds between the Unix Epoch of January 1 1970 00:00:00 GMT and the time specified. You can't omit them out of order though. Example 11-19 checks whether the date is valid, and then creates a timestamp.

Example 11-19. Creating a timestamp from the components of a date

 <?php   echo("Validating: 5/31/2005<br>");   if (checkdate(5,31,2005)) {     echo('Date accepted: ');     $new_date=mktime(18,05,35,5,31,2005);     echo date("r",$new_date);   }   else {     echo ('Invalid date.');   }   echo("<br>"); ?> 

When run, this code produces the screen shown in Figure 11-16.

Figure 11-16. A timestamp created from its components

Now that we've covered dates and times, it's time to head onto more exciting topics. Note that the number of the year might be a two- or four-digit value, between 0 to 69 mapping to 2000 to 2069 and 70 to 100 to 1970 to 2000. On systems where time_t is a 32-bit signed integer, which is most common today, the valid range for the year is somewhere between 1901 and 2038. This limitation is fixed since the release of PHP 5.1.0.