It seems that the answer to this issue is located here:
'With the advent of PHP 5.2, there are object representations of the date and
timezone, named DateTime and DateTimeZone respectively. You can see the methods and constants available to the new classes by running'
We've renamed this class to SSDatetime in the latest release, due out shortly. The old Datetime class still exists, but is only defined if PHP doesn't already define it. That way, legacy code written for PHP 5.1 will still work if it used Datetime (however, it won't if the server is upgraded to 5.2 - you'll have to change to using SSDatetime for it to work then).