[fanf2]

See the definition of seconds since the epoch at https://pubs.opengroup.org/onlinepubs/9699919799.2018edition...

  tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
      (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
      ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400