"unix" timestamp is number of seconds since Jan 1, 1970 UTC.
This allows scripts that use templates to generate time
deltas in seconds if desired from state attributes such
as last_updated.
Some examples:
timestamp now is
{{ as_timestamp(now) }}
timstamp of last change is
{{ as_timestamp(states.binary_sensor.garage_door.last_changed) }}
seconds since last change is
{{ as_timestamp(now) - as_timestamp(states.binary_sensor.garage_door.last_changed) }}
Daniel Høyer Iversen
Robbie Trencheny
Charles Spirakis
Paulus Schoutsen
Fabian Affolter