Class to represent a mutable date and time
I guess I am not really seeing what value this class adds above and beyond what DateTime and related classes give to you "out of the box".

If you need a method or two to perhaps format a date in some specific way to be compatible with Amazon, then perhaps you could extend the base DateTime class with these methods.

Even then, I still question the value of introducing a whole class to, in essence, do these sorts of trivial things:

$datetime = new DateTime();
$datetime-&gt;modify('-1 month');
$datetime-&gt;modify('-10 days');
$amazonFormat = $datetime-&gt;format('c');




It is odd how you are working with PHP's procedural date functions interchangeably with DateTime.

For example in your constructor you write this:

public function __construct()
{
    date_default_timezone_set('America/Chicago');
    $date = new DateTime(date('Y/m/d H:i:s'));
    $this-&gt;setDate($date);
}


Which if you truly wanted to use DateTime might look like this:

public function __construct()
{
    $datetime = new DateTime('now', new DateTimeZone('America/Chicago'));
    $this-&gt;setDate($date);
}


There is no reason to change your system's default timezone on the fly if you just need to apply a timezone to one specific DateTime object. Right now your code could actually severely break other code in your system that follows instantiation of one of these objects, in that you have munged with default setting for all code that follows in script execution.  If the default setting on the server were for a different time zone,  all code execution up to the point of instantiation would operate in that timezone, while all code after would operate in America/Chicago.

If you truly are trying to set this as your default timezone for your entire application, this function call should be made somewhere in your bootstrapping process.

If this truly is a class-level setting, should the America/Chicago (or similar) value be passed to the constructor, or be a class constant? Right now you are "hiding" this setting inside method logic, which is a bad idea.



Why even have your currentDate() method?  It exactly the same as the constructor logic.



Your method names seem odd and could potentially cause confusion.  get/setDate() are really getting/setting DateTime objects which is not clear from the method names.  Your setDate() has an additional problem in that it does not enforce that a DateTime object be passed via type hinting, so some confused developer could pass a string or some other value to this method and put your object in a bad state.



You have the general problem throughout your public methods in that you are not enforcing data types for parameters at all.  This is typically achieved either through type-hinting (if applicable) or through validation within the method if type-hinting is not available for a given data type (which will vary based on version of PHP you are designing against).