Sudheer Satyanarayana has provided a handy cheat sheet for those using the Zend_Date component of the Zend Framework listing the default values of the constants it provides.
Date and time handling in general is a problem in programming. For PHP programmers, there's a good library out there that performs all the difficult tasks and provides convenient APIs. Zend_Date has several constants defined. It is good to know what each one of them represents.