You should make use of the DateTime and DateInterval classes in Php, and things will turn to be very easy and readable.
Example: Lets get the previous day.
// always make sure to have set your default timezone
date_default_timezone_set('Europe/Berlin');
// create DateTime instance, holding the current datetime
$datetime = new DateTime();
// create one day interval
$interval = new DateInterval('P1D');
// modify the DateTime instance
$datetime->sub($interval);
// display the result, or print_r($datetime); for more insight
echo $datetime->format('Y-m-d');
/**
* TIP:
* if you dont want to change the default timezone, use
* use the DateTimeZone class instead.
*
* $myTimezone = new DateTimeZone('Europe/Berlin');
* $datetime->setTimezone($myTimezone);
*
* or just include it inside the constructor
* in this form new DateTime("now", $myTimezone);
*/
References: Modern PHP, New Features and Good Practices By Josh Lockhart