What date/time is UNIX time calculated based off of?

What date/time is UNIX time calculated based off of?

• 00:00:00 31 December 1967
• 00:00:00 1 January 0000
• 00:00:00 1 January, 1900
• 00:00:00 1 January 1970 

 

EXPLANATION

 

Epoch time, also known as Unix time, is defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970 (not counting leap seconds).