time_t was almost certainly 64-bit on 64-bit archs from the beginning. There's just a lot of 32-bit stuff still out there. And here and there a disk format standarized before people started thinking about the end of time.
Up to ext3 are using 32-bit timestamps. Ext4 now uses 34-bit, by extending only the upper range, and can cope with 7 * 2^32 seconds after the epoch, which is around 2446.
HFS+ uses a different 32-bit timestamp, which is unsigned and starts from 1904-01-01, so it expires on 2040.
