For PHP developers - date(), mktime() and strtotime() (as well as other functions) use 32bit signed integers for storing the date representation (except in some instances of a 64bit version of PHP on a 64bit OS), which is not y2k38 safe. It is preferred to use the DateTime class available in 5.2 and above which stores the date in a 64bit signed integer. There are other reasons to use the DateTime class as well - and it should generally be best practice to move away from date(), mktime(), strtotime() and other similiar functionality.