This website has a calculator:
It is the number of seconds since January 1st, 1970 at midnight GMT.
The 11, is because something took 11 seconds.
Here’s a post explaining why 1970 was chosen along with the definition:
- Unix time, or POSIX time, is a system for describing points in time, defined as the number of seconds elapsed since midnight proleptic Coordinated Universal Time (UTC) of January 1, 1970, not counting leap seconds.
Why are dates calculated from January 1st, 1970?
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solved System Time is 10 Digit , How? [closed]