Clone the date with the clone operator:
$date1 = new DateTime();
$date2 = clone $date1;
Clones are shallow by default, but deep enough for a DateTime. In your own objects, you can define the
__clone() magic method to clone the properties (i.e. child objects) that make sense to be cloned when the parent object changes.
(I'm not sure why the documentation thinks a good example of needing to clone an object is GTK. Who uses GTK in PHP?)