This:

  echo strftime("%Y-%m-%d %H:%M:%S", -200000);

outputs an empty string. However, this:

  $foo = localtime(-200000000); echo $foo[5]+1900;

outputs "1963" in Linux 2.2.15, but outputs an empty string in Win2k. While negative time_t is undefined according to POSIX, time functions should probably work consistently, and strftime() should never output nothing. (That is, since there is no error condition documented for strftime(), this:

  echo strftime("HONK", -200000);

should /always/ output "HONK", even if the %-values end up empty because of the undefined meaning of negative time values.)

So, either strftime with negative time values should work,
or localtime with negative time values should fail (and the rest of the time functions made consistent with either decision). If it's decided that negative time values are to cause the functions to fail, then error conditions should probably be defined or documented for those functions (and maybe even warnings?).

(To be clear, that's not "PHP is nonconforming", but rather "undocumented and nonportable inconsistency amongst *time functions is a particularly icky form of nasal demon".)