public static class MonthExtensions
{
    /// 

    /// Gets the first day of the month.
    /// 
    /// The given date.
    /// the first day of the month
    public static DateTime GetFirstDayOfMonth(DateTime givenDate)
    {
        return new DateTime(givenDate.Year, givenDate.Month, 1);
    }

    /// 

    /// Gets the last day of month.
    /// 
    /// The given date.
    /// the last day of the month
    public static DateTime GetTheLastDayOfMonth(DateTime givenDate)
    {
        return GetFirstDayOfMonth(givenDate).AddMonths(1).Subtract(new TimeSpan(1, 0, 0, 0, 0));
    }
}

    public static int GetAge(DateTime birthday)
    {
        DateTime now = DateTime.Today;
        int age = now.Year - birthday.Year;
        if (now < birthday.AddYears(age)) age--;

        return age;
    }

var unix_timestamp = Math.round((new Date()).getTime() / 1000);

function getAge($birth_date) {
    return floor((time() - strtotime($birth_date)) / 31556926);
}

function getZodiacSign($birth_date) {
    $bd = explode("-", $birth_date);
    $daymonth = $bd[1] . "-" . $bd[2];

    if ($daymonth >= '01-21' and $daymonth <= '02-20') {
        $zodiac = 'Aquarius';
    } elseif ($daymonth >= '02-21' and $daymonth <= '03-20') {
        $zodiac = 'Pisces';
    } elseif ($daymonth >= '03-21' and $daymonth <= '04-20') {
        $zodiac = 'Aries';
    } elseif ($daymonth >= '04-21' and $daymonth <= '05-20') {
        $zodiac = 'Taurus';
    } elseif ($daymonth >= '05-21' and $daymonth <= '06-21') {
        $zodiac = 'Gemini';
    } elseif ($daymonth >= '06-22' and $daymonth <= '07-22') {
        $zodiac = 'Cancer';
    } elseif ($daymonth >= '07-23' and $daymonth <= '08-23') {
        $zodiac = 'Leo';
    } elseif ($daymonth >= '08-24' and $daymonth <= '09-23') {
        $zodiac = 'Virgo';
    } elseif ($daymonth >= '09-24' and $daymonth <= '10-23') {
        $zodiac = 'Libra';
    } elseif ($daymonth >= '10-24' and $daymonth <= '11-22') {
        $zodiac = 'Scorpio';
    } elseif ($daymonth >= '11-23' and $daymonth <= '12-21') {
        $zodiac = 'Sagittarius';
    } else {
        $zodiac = 'Capricorn';
    }
    return $zodiac;
}

//Will return "Aquarius"
echo getZodiacSign('1982-01-27');

function humanTiming ($time)
{
        $time = strtotime($time);
    $time = time() - $time; // to get the time since that moment

    $tokens = array (
        31536000 => 'year',
        2592000 => 'month',
        604800 => 'week',
        86400 => 'day',
        3600 => 'hour',
        60 => 'minute',
        1 => 'second'
    );

    foreach ($tokens as $unit => $text) {
        if ($time < $unit) continue;
        $numberOfUnits = floor($time / $unit);
        return $numberOfUnits.' '.$text.(($numberOfUnits>1)?'s':'').' ago';
    }

}

$time = '2013-05-25 15:45:22';
echo 'Event happened '.humanTiming($time);

ini_set('session.save_path', $_SERVER['DOCUMENT_ROOT'] . '/sessions/'); //Important!
ini_set('session.gc_maxlifetime', 86400);
ini_set('session.cookie_lifetime', 86400);

# DO:
2.hours.ago # => Fri, 02 Mar 2012 20:04:47 JST +09:00
1.day.from_now # => Fri, 03 Mar 2012 22:04:47 JST +09:00
Date.today.to_time_in_current_zone # => Fri, 02 Mar 2012 22:04:47 JST +09:00
Date.current # => Fri, 02 Mar
Time.zone.parse("2012-03-02 16:05:37") # => Fri, 02 Mar 2012 16:05:37 JST +09:00
Time.zone.now # => Fri, 02 Mar 2012 22:04:47 JST +09:00
Time.current # Same thing but shorter. (Thank you Lukas Sarnacki pointing this out.)
Time.zone.today # If you really can't have a Time or DateTime for some reason
Time.zone.now.utc.iso8601 # When supliyng an API (you can actually skip .zone here, but I find it better to always use it, than miss it when it's needed)
Time.strptime(time_string, '%Y-%m-%dT%H:%M:%S%z').in_time_zone(Time.zone) # If you can't use Time#parse
14:27:45
# DON’Ts
Time.now # => Returns system time and ignores your configured time zone.
Time.parse("2012-03-02 16:05:37") # => Will assume time string given is in the system's time zone.
Time.strptime(time_string, '%Y-%m-%dT%H:%M:%S%z') # Same problem as with Time#parse.
Date.today # This could be yesterday or tomorrow depending on the machine's time zone.
Date.today.to_time # => # Still not the configured time zone.

$startDate = 'now';
$endDate = '2013-08-12 14:30:00';

function date_diff($date1, $date2) {
    $diff = strtotime($date2) - strtotime($date1);
    return abs($diff);
}

function seconds2human($ss) {
    $s = $ss % 60;
    $m = floor(($ss % 3600) / 60);
    $h = floor(($ss) / 3600);

    return "$h hours, $m minutes, $s seconds";
}

echo "This file will expire in  ". seconds2human(date_diff($startDate,$endDate));

$time_start = microtime(true);

// your PHP code here

$time_end = microtime(true);
$time = $time_end - $time_start;

echo 'This script has been executed in ' . $time . ' sec.'; 

//$row[create_datetime] == '2013-05-06 03:14:43';
//$formatted_datetime == '06.05.2013 03:14';

define('SHORTDATETIME', 'd.m.Y H:i');
$formatted_datetime = date_format(date_create($row[create_datetime]), SHORTDATETIME);

SELECT COUNT(id) FROM my_table WHERE created_datetime >= (DATE_FORMAT(DATE_ADD(NOW(), INTERVAL -1 DAY), '%Y-%m-%d')) AND created_datetime < (DATE_FORMAT(NOW(), '%Y-%m-%d'))

SELECT COUNT(id) FROM my_table WHERE date_format(created_datetime, '%Y%m') = date_format(now(), '%Y%m') 

function sql_to_rfc822($sql_date_str) {
    $d1 = explode(" ", $sql_date_str);
    $d2 = explode("-", $d1[0]);
    $d3 = explode(":", $d1[1]);
    $rfc822_date_str = date("D, d M Y H:i:s", mktime($d3[0], $d3[1], $d3[2], $d2[1], $d2[2], $d2[0])) . "  GMT";
    return $rfc822_date_str;
}