January 1st 1970 is the beginning of the UNIX epoch. Where dates are not being handled correctly, and are returned from the database as zero (most normally where the field name or type holding the date has changed but the code has not been updated) this will be parsed as January 1st 1970.
Storing dates as timestamps uses a large integer representing the number of seconds since 01.01.1970. It's actually a really convenient way of managing dates because you can manipulate them (for time zones) really easily.