This equation gives you the Unix Time in seconds when you input a Gregorian (standard modern calendar date).  Note this converts a date you in put as Day, Month, Year into the number of seconds after the Unix Time epoch that corresponds to Midnight that start that the input date.

The Unix Time epoch is midnight on January 1st, 1970 -- which is 00:00:00 UTC on 1 January 1970.

"Unix time (also known as POSIX time or Epoch time) is a system for describing instants in time, 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. It is used widely in Unix-like and many other operating systems and file formats. Because it does not handle leap seconds, it is neither a linear representation of time nor a true representation of UTC. Unix time may be checked on most Unix systems by typing date +%s on the command line."¹ 

Example

If you input January 1, 1970, this equation will return the Unix Time:  0 seconds

If you input September 16, 2004, this equation will return the Julian Day: 1095292800 seconds

See Also

• Gregorian Calendar to Julian Day Number (JDN)
• Modified Julian Day (MJD)
• Truncated Julian Day

1. ^ https://en.wikipedia.org/wiki/Unix_time