Originariamente inviato da Alhazred
Non lo so di preciso, ma forse ha a che fare col fatto che 64 è precedente al 71 che è l'anno di riferimento per i timestamp.
Non so se è davvero così, ma è un'ipotesti.
ipotesi corretta.

dal manuale php:


Note:
If the number of the year is specified in a two digit format, the values between 00-69 are mapped to 2000-2069 and 70-99 to 1970-1999. See the notes below for possible differences on 32bit systems (possible dates might end on 2038-01-19 03:14:07).

Note:

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.

For 64-bit versions of PHP, the valid range of a timestamp is effectively infinite, as 64 bits can represent approximately 293 billion years in either direction.