El problema que obtienes sucede sólo en PHP bajo windows ..
Cita: Year may be a two or four digit value, with values between 0-69 mapping to 2000-2069 and 70-99 to 1970-1999 (on systems where time_t is a 32bit signed integer, as most common today, the valid range for year is somewhere between 1901 and 2038).
Windows: Negative timestamps are not supported under any known version of Windows. Therefore the range of valid years includes only 1970 through 2038.
www.php.net/mktime
(sólo es para que sepas por qué sucede en Windows este efecto).
Un saludo,