title: Math to convert seconds since 1970 into date and vice versa
body: <p>I have seconds since Jan 1 1970 00:00 as an int64 in nanoseconds and I'm trying to convert it into month/day/year/day of week.</p>
  
  <p>It's easy to do this iteratively, I have that working but I want to do it formulaically. I'm looking for the actual math.</p>