How to get elapsed seconds since Jan 1, 1970? I am building a time-critical system using Arduino. Is there any way to get the elapsed seconds like time.h of C in Arduino? I tried time(NULL) but it returns 0. Then moved forward to use now() in Time.h. But the result persists. 

How can I get the elapsed seconds since Jan 1, 1970 until now?
An arduino does not have a real time clock (RTC) built in. Every time it restarts it will reset the millis counter.

You will need to add a RTC peripheral that you can poll to find the current date and time.