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1. When the Big Bang occured according to a Unix computer system. This date is when the first Unix computer was made operable. If you find a file that was from Wed Dec 31, 1969 on your Linux PC, you can be sure that the date on that file got knackered.

2. A date to utter when something hasn't been done in a long time, referring to the idea that January 1, 1970 is the "beginning of time". Used when something seems like it hasn't been done since the beginning of time.
1. On January 1, 1970 Unix was running on a mainframe computer.

2. Romeo:Man, you stink! When was the last time you showered?
Joliet:I took a shower on January 1, 1970.

2. The last time the gas station changed the coffee in the coffeepot on January 1, 1970.