A thread at WebmasterWorld named Google Cache Date: Dec 31, 1969 is proof that Google uses Unix time stamps for the cache database. Dec 31, 1969 is the default value for a time stamp in Unix. For many operating systems, it is the earliest time you can set a clock back to. It is the second before the epoch, the date used as the "beginning of time" for timestamps. Time values in Unix systems are represented as seconds since the epoch, with library functions available for converting these values into standard date and time formats. The epoch on Unix and POSIX systems is 1970-01-01 00:00:00 UTC.
Interesting find, wonder what went wrong at Google.