Reset date for Cumulative Statistics System when not in table

I am new to Postgres and started reading about the The Cumulative Statistics System. 

When I came across pg_statio_all_tables I was wondering what is the reference date for the counters like (heap_blks_read, heap_blks_hit ... ) This table does not have a stats_reset column like others. 

So are this numbers accumulating from the creation of the table or since the startup of the cluster ?

Maybe someone can drop me a line.

cheers

Guenter