Uses Unix timestamp as base unit (seconds since 1970-01-01, 00:00 GMT)

Only 32 bit signed integers for timestamps (1901 to 2038)

Limited to only positive numbers on some Operating Systems (1970 to 2038)

strtotime() is buggy and its implementation is very complex

No way of dealing correctly with timezones

Some functions are Operating System dependent

Avoid functions that deal with timestamps as much as you can!