From the manual:
The valid range of a timestamp is typically from Fri, 13 Dec 1901
20:45:54 UTC to Tue, 19 Jan 2038 03:14:07 UTC.
(These are the dates that correspond to the minimum and maximum values for a 32-bit signed
integer.)
Additionally, not all platforms support negative timestamps,
therefore your date range may be limited to no earlier than the Unix
epoch. This means that e.g. dates prior to Jan 1, 1970 will not work
on Windows, some Linux distributions, and a few other operating
systems. PHP 5.1.0 and newer versions overcome this limitation though.
So it depends on the platform you're running it on. It needs to be 64 bit to exceed the range stated in the manual.