There are nearly fifty date and time functions, so for the purpose of this tutorial we will narrow them down to three.

Function	Description
time()	Returns the Current Time as a Unix Timestamp
date()	Formats a Local Time/Date
strtotime()	Parses an English Textual Date or Time Into a Unix Timestamp

Since we will be using the word "timestamp" throughout this page, we will begin by explaining what it is. A Unix Timestamp is the number of seconds that have passed since January 1, 1970. Currently, the timestamp is 1732179024, but since it changes every second, the number will increase if you refresh the page. (The timestamp reflects the time set on the server, and not on the client's computer.)

The PHP Time() Function

The time() function returns the current timestamp. The timestamp that you see in the previous paragraph is generated by a very small line of code: <?php echo time(); ?>

The time() function can also return a modified timestamp. You can add or subtract any number of seconds to the function in order to return the timestamp of a previous or upcoming date and time.

For example, to return a timestamp for next week, or to return a timestamp from last week, I can add or subtract 7 days by determining how many seconds are involved (7 days * 24 hours per day * 60 minutes in an hour * 60 seconds in an hour = number of second in 7 days).

<?php
  $last_week = time() - (7 * 24 * 60 * 60);
  $next_week = time() + (7 * 24 * 60 * 60);
  $next_month = time() + (30 * 24 * 60 * 60);
  echo "Last Week: " . $last_week . "<br />";
  echo "Next Week: " . $next_week . "<br />";
  echo "Next Month: " . $next_month . "<br />";
?>

The result will be:

Last Week: 1731574224
Next Week: 1732783824
Next Month: 1734771024


The PHP Strtotime() Function

The strtotime() function accepts an English datetime description and turns it into a timestamp. It is a simple way to determine "next week" or "last monday" without using the time() function and a bunch of math.

Some examples are:

<?php
  echo strtotime("now") . "<br />";
  echo strtotime("tomorrow") . "<br />";
  echo strtotime("yesterday") . "<br />";
  echo strtotime("10 September 2000") . "<br />";
  echo strtotime("+1 day") . "<br />";
  echo strtotime("+1 week") . "<br />";
  echo strtotime("+1 week 2 days 4 hours 2 seconds") . "<br />";
  echo strtotime("next Thursday") . "<br />";
  echo strtotime("last Monday") . "<br />";
  echo strtotime("4pm + 2 Hours") . "<br />";
  echo strtotime("now + 2 fortnights") . "<br />";
  echo strtotime("last Monday") . "<br />";
  echo strtotime("2pm yesterday") . "<br />";
  echo strtotime("7am 12 days ago") . "<br />";
?>

The PHP Date() Function

The date() function formats a timestamp so that it actually makes sense, such as 2:50 AM Thursday, November 21, 2024.

The date() function accepts two arguments, according to the following syntax: date(format, timestamp);. The first argument is the format that you want the timestamp in. The second argument (optional) is the timestamp that you want formatted. If no timestamp is supplied, the current timestamp will be used.

PHP provides over thirty-five case-sensitive characters that are used to format the date and time. These characters are:

	Character	Description	Example
Day	j	Day of the Month, No Leading Zeros	1 - 31
Day	d	Day of the Month, 2 Digits, Leading Zeros	01 - 31
Day	D	Day of the Week, First 3 Letters	Mon - Sun
Day	l (lowercase 'L')	Day of the Week	Sunday - Saturday
Day	N	Numeric Day of the Week	1 (Monday) - 7 (Sunday)
Day	w	Numeric Day of the Week	0 (Sunday) - 6 (Saturday)
Day	S	English Suffix For Day of the Month	st, nd, rd or th
Day	z	Day of the Year	0 - 365
Week	W	Numeric Week of the Year (Weeks Start on Mon.)	1 - 52
Month	M	Textual Representation of a Month, Three Letters	Jan - Dec
Month	F	Full Textual Representation of a Month	January - December
Month	m	Numeric Month, With Leading Zeros	01 - 12
Month	n	Numeric Month, Without Leading Zeros	1 - 12
Month	t	Number of Days in the Given Month	28 - 31
Year	L	Whether It's a Leap Year	Leap Year: 1, Otherwise: 0
Year	Y	Numeric Representation of a Year, 4 Digits	1999, 2003, etc.
Year	y	2 Digit Representation of a Year	99, 03, etc.
Time	a	Lowercase Ante Meridiem & Post Meridiem	am or pm
Time	A	Uppercase Ante Meridiem & Post Meridiem	AM or PM
Time	B	Swatch Internet Time	000 - 999
Time	g	12-Hour Format Without Leading Zeros	1 - 12
Time	G	24-Hour Format Without Leading Zeros	0 - 23
Time	h	12-Hour Format With Leading Zeros	01 - 12
Time	H	24-Hour Format With Leading Zeros	00 - 23
Time	i	Minutes With Leading Zeros	00 - 59
Time	s	Seconds With Leading Zeros	00 - 59
Timezone	e	Timezone Identifier	Example: UTC, Atlantic
Timezone	I (capital i)	Whether Date Is In Daylight Saving Time	1 if DST, otherwise 0
Timezone	O	Difference to Greenwich Time In Hours	Example: +0200
Timezone	P	Difference to Greenwich Time, With Colon	Example: +02:00
Timezone	T	Timezone Abbreviation	Examples: EST, MDT ...
Timezone	Z	Timezone Offset In Seconds	-43200 through 50400

Using a combination of these characters and commas, periods, dashes, semicolons and backslashes, you can now format dates and times.

<?php
  // Will Echo: 2:50 AM Thursday, November 21, 2024
  echo date("g:i A l, F d, Y");

  // Will Echo: 2024-11-20
  $yesterday = strtotime("yesterday");
  echo date("Y-m-d", $yesterday);
?>

Calculating Past & Future Dates

It is very simple to compare two dates in php, to verify if one is past, present or future.

Since all dates can be converted back into timestamps, all it takes is a simple comparison of the two timestamps, which are essentially numbers. If the date in question is a bigger number than the current date, than you know that your date is in the future, and if the date is smaller than the current date, you know that the date is already past.

<?php
  $my_date = strtotime("10 April 2005");
  if(strtotime($my_date) > time()) {
    echo "Future Date!";
  } if(strtotime($my_date) < time()) {
    echo "Past Date!";
  } if(strtotime($my_date) == time()) {
    echo "Current Date!";
  }
?>