The PHP timestamp is a measure of the number of seconds (or microseconds with microtime) that have elapsed since the Unix Epoch (January 1 1970 00:00:00 GMT). So if you want to work with dates before the Epoch you will have to cater for that in your code. It also cannot measure dates in the future that require more bits then allocated for its datatype - not sure what that date is exactly.
Therefore, normally the PHP time functions are used to get the current date, and make calculations. But if you want to save the resulting date, its better to save it as an actual date if its range may lie outside the limitation of the PHP timestamp.