PHP Date Function

<?php
$tomorrow = mktime(0, 0, 0, date("m"), date("d")+1, date("y"));
echo "Tomorrow is ".date("m/d/y", $tomorrow); 
?>

Important Full Date and Time:

• r: Displays the full date, time and timezone offset. It is equivalent to manually entering date("D, d M Y H:i:s O")

Time:

• a: am or pm depending on the time
• A: AM or PM depending on the time
• g: Hour without leading zeroes. Values are 1 through 12.
• G: Hour in 24-hour format without leading zeroes. Values are 0 through 23.
• h: Hour with leading zeroes. Values 01 through 12.
• H: Hour in 24-hour format with leading zeroes. Values 00 through 23.
• i: Minute with leading zeroes. Values 00 through 59.
• s: Seconds with leading zeroes. Values 00 through 59.

Day:

• d: Day of the month with leading zeroes. Values are 01 through 31.
• j: Day of the month without leading zeroes. Values 1 through 31
• D: Day of the week abbreviations. Sun through Sat
• l: Day of the week. Values Sunday through Saturday
• w: Day of the week without leading zeroes. Values 0 through 6.
• z: Day of the year without leading zeroes. Values 0 through 365.

Month:

• m: Month number with leading zeroes. Values 01 through 12
• n: Month number without leading zeroes. Values 1 through 12
• M: Abbreviation for the month. Values Jan through Dec
• F: Normal month representation. Values January through December.
• t: The number of days in the month. Values 28 through 31.

Year:

• L: 1 if it's a leap year and 0 if it isn't.
• Y: A four digit year format
• y: A two digit year format. Values 00 through 99.

Other Formatting:

• U: The number of seconds since the Unix Epoch (January 1, 1970)
• O: This represents the Timezone offset, which is the difference from Greenwich Meridian Time (GMT). 100 = 1 hour, -600 = -6 hours

Starting a PHP Session

<?php
session_start(); 
$_SESSION['views'] = 1; // store session data
echo "Pageviews = ". $_SESSION['views']; //retrieve data
?>

<?php
session_start(); 
session_destroy();
?>

PHP Cookies - Background

<?php
if(isset($_COOKIE['lastVisit']))
	$visit = $_COOKIE['lastVisit']; 
else
	echo "You've got some stale cookies!";

echo "Your last visit was - ". $visit;
?>

Your last visit was - 11:48 - 02/28/08 

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.lang.time;

import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

/**
 * <p>Date and time formatting utilities and constants.</p>
 *
 * <p>Formatting is performed using the
 * {@link org.apache.commons.lang.time.FastDateFormat} class.</p>
 *
 * @author Apache Ant - DateUtils
 * @author <a href="mailto:sbailliez@apache.org">Stephane Bailliez</a>
 * @author <a href="mailto:stefan.bodewig@epost.de">Stefan Bodewig</a>
 * @author Stephen Colebourne
 * @author <a href="mailto:ggregory@seagullsw.com">Gary Gregory</a>
 * @since 2.0
 * @version $Id: DateFormatUtils.java 437554 2006-08-28 06:21:41Z bayard $
 */
public class DateFormatUtils {

    /**
     * ISO8601 formatter for date-time without time zone.
     * The format used is <tt>yyyy-MM-dd'T'HH:mm:ss</tt>.
     */
    public static final FastDateFormat ISO_DATETIME_FORMAT
            = FastDateFormat.getInstance("yyyy-MM-dd'T'HH:mm:ss");

    /**
     * ISO8601 formatter for date-time with time zone.
     * The format used is <tt>yyyy-MM-dd'T'HH:mm:ssZZ</tt>.
     */
    public static final FastDateFormat ISO_DATETIME_TIME_ZONE_FORMAT
            = FastDateFormat.getInstance("yyyy-MM-dd'T'HH:mm:ssZZ");

    /**
     * ISO8601 formatter for date without time zone.
     * The format used is <tt>yyyy-MM-dd</tt>.
     */
    public static final FastDateFormat ISO_DATE_FORMAT
            = FastDateFormat.getInstance("yyyy-MM-dd");

    /**
     * ISO8601-like formatter for date with time zone.
     * The format used is <tt>yyyy-MM-ddZZ</tt>.
     * This pattern does not comply with the formal ISO8601 specification
     * as the standard does not allow a time zone  without a time.
     */
    public static final FastDateFormat ISO_DATE_TIME_ZONE_FORMAT
            = FastDateFormat.getInstance("yyyy-MM-ddZZ");

    /**
     * ISO8601 formatter for time without time zone.
     * The format used is <tt>'T'HH:mm:ss</tt>.
     */
    public static final FastDateFormat ISO_TIME_FORMAT
            = FastDateFormat.getInstance("'T'HH:mm:ss");

    /**
     * ISO8601 formatter for time with time zone.
     * The format used is <tt>'T'HH:mm:ssZZ</tt>.
     */
    public static final FastDateFormat ISO_TIME_TIME_ZONE_FORMAT
            = FastDateFormat.getInstance("'T'HH:mm:ssZZ");

    /**
     * ISO8601-like formatter for time without time zone.
     * The format used is <tt>HH:mm:ss</tt>.
     * This pattern does not comply with the formal ISO8601 specification
     * as the standard requires the 'T' prefix for times.
     */
    public static final FastDateFormat ISO_TIME_NO_T_FORMAT
            = FastDateFormat.getInstance("HH:mm:ss");

    /**
     * ISO8601-like formatter for time with time zone.
     * The format used is <tt>HH:mm:ssZZ</tt>.
     * This pattern does not comply with the formal ISO8601 specification
     * as the standard requires the 'T' prefix for times.
     */
    public static final FastDateFormat ISO_TIME_NO_T_TIME_ZONE_FORMAT
            = FastDateFormat.getInstance("HH:mm:ssZZ");

    /**
     * SMTP (and probably other) date headers.
     * The format used is <tt>EEE, dd MMM yyyy HH:mm:ss Z</tt> in US locale.
     */
    public static final FastDateFormat SMTP_DATETIME_FORMAT
            = FastDateFormat.getInstance("EEE, dd MMM yyyy HH:mm:ss Z", Locale.US);

    //-----------------------------------------------------------------------
    /**
     * <p>DateFormatUtils instances should NOT be constructed in standard programming.</p>
     *
     * <p>This constructor is public to permit tools that require a JavaBean instance
     * to operate.</p>
     */
    public DateFormatUtils() {
        super();
    }

    /**
     * <p>Formats a date/time into a specific pattern using the UTC time zone.</p>
     *
     * @param millis  the date to format expressed in milliseconds
     * @param pattern  the pattern to use to format the date
     * @return the formatted date
     */
    public static String formatUTC(long millis, String pattern) {
        return format(new Date(millis), pattern, DateUtils.UTC_TIME_ZONE, null);
    }

    /**
     * <p>Formats a date/time into a specific pattern using the UTC time zone.</p>
     *
     * @param date  the date to format
     * @param pattern  the pattern to use to format the date
     * @return the formatted date
     */
    public static String formatUTC(Date date, String pattern) {
        return format(date, pattern, DateUtils.UTC_TIME_ZONE, null);
    }
   
    /**
     * <p>Formats a date/time into a specific pattern using the UTC time zone.</p>
     *
     * @param millis  the date to format expressed in milliseconds
     * @param pattern  the pattern to use to format the date
     * @param locale  the locale to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String formatUTC(long millis, String pattern, Locale locale) {
        return format(new Date(millis), pattern, DateUtils.UTC_TIME_ZONE, locale);
    }

    /**
     * <p>Formats a date/time into a specific pattern using the UTC time zone.</p>
     *
     * @param date  the date to format
     * @param pattern  the pattern to use to format the date
     * @param locale  the locale to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String formatUTC(Date date, String pattern, Locale locale) {
        return format(date, pattern, DateUtils.UTC_TIME_ZONE, locale);
    }
   
    /**
     * <p>Formats a date/time into a specific pattern.</p>
     *
     * @param millis  the date to format expressed in milliseconds
     * @param pattern  the pattern to use to format the date
     * @return the formatted date
     */
    public static String format(long millis, String pattern) {
        return format(new Date(millis), pattern, null, null);
    }

    /**
     * <p>Formats a date/time into a specific pattern.</p>
     *
     * @param date  the date to format
     * @param pattern  the pattern to use to format the date
     * @return the formatted date
     */
    public static String format(Date date, String pattern) {
        return format(date, pattern, null, null);
    }
   
    /**
     * <p>Formats a date/time into a specific pattern in a time zone.</p>
     *
     * @param millis  the time expressed in milliseconds
     * @param pattern  the pattern to use to format the date
     * @param timeZone  the time zone  to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String format(long millis, String pattern, TimeZone timeZone) {
        return format(new Date(millis), pattern, timeZone, null);
    }

    /**
     * <p>Formats a date/time into a specific pattern in a time zone.</p>
     *
     * @param date  the date to format
     * @param pattern  the pattern to use to format the date
     * @param timeZone  the time zone  to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String format(Date date, String pattern, TimeZone timeZone) {
        return format(date, pattern, timeZone, null);
    }

    /**
     * <p>Formats a date/time into a specific pattern in a locale.</p>
     *
     * @param millis  the date to format expressed in milliseconds
     * @param pattern  the pattern to use to format the date
     * @param locale  the locale to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String format(long millis, String pattern, Locale locale) {
        return format(new Date(millis), pattern, null, locale);
    }

    /**
     * <p>Formats a date/time into a specific pattern in a locale.</p>
     *
     * @param date  the date to format
     * @param pattern  the pattern to use to format the date
     * @param locale  the locale to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String format(Date date, String pattern, Locale locale) {
        return format(date, pattern, null, locale);
    }

    /**
     * <p>Formats a date/time into a specific pattern in a time zone  and locale.</p>
     *
     * @param millis  the date to format expressed in milliseconds
     * @param pattern  the pattern to use to format the date
     * @param timeZone  the time zone  to use, may be <code>null</code>
     * @param locale  the locale to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String format(long millis, String pattern, TimeZone timeZone, Locale locale) {
        return format(new Date(millis), pattern, timeZone, locale);
    }

    /**
     * <p>Formats a date/time into a specific pattern in a time zone  and locale.</p>
     *
     * @param date  the date to format
     * @param pattern  the pattern to use to format the date
     * @param timeZone  the time zone  to use, may be <code>null</code>
     * @param locale  the locale to use, may be <code>null</code>
     * @return the formatted date
     */
    public static String format(Date date, String pattern, TimeZone timeZone, Locale locale) {
        FastDateFormat df = FastDateFormat.getInstance(pattern, timeZone, locale);
        return df.format(date);
    }

}

ISO 8601 advises numeric representation of dates and times on an internationally agreed basis. It represents elements from the largest to the smallest element: year-month-day:

• Calendar date is the most common date representation. It is:

YYYY-MM-DD

where YYYY is the year in the Gregorian calendar, MM is the month of the year between 01 (January) and 12 (December), and DD is the day of the month between 01 and 31.

Example: 2003-04-01 represents the first day of April in 2003.

• Week date is an alternative date representation used in many commercial and industrial applications. It is:

YYYY-Www-D

where YYYY is the Year in the Gregorian calendar, ww is the week of the year between 01 (the first week) and 52 or 53 (the last week), and D is the day in the week between 1 (Monday) and 7 (Sunday).

Example: 2003-W14-2 represents the second day of the fourteenth week of 2003.

• Time of the day is the time representation, using the 24-hour timekeeping system. It is:

hh:mm:ss

where hh is the number of complete hours that have passed since midnight, mm is the number of complete minutes since the start of the hour, and ss is the number of complete seconds since the start of the minute.

Example: 23:59:59 represents the time one second before midnight.

• Date and time represents a specified time of a specified day. When use is made of the calendar date the representation is:

YYYY-MM-DDThh:mm:ss

where the capital letter T is used to separate the date and time components. Thus, for a very precise date and time, look at this:

Example: 2003-04-01T13:01:02 represents one minute and two seconds after one o'clock in the afternoon of 2003-04-01.

The standard has provisions for:

• the omission of components representing smaller units (seconds, minutes), where such precision is not needed,
• the addition of a decimal fraction to the smallest time unit where higher precision is needed.