datetime length derivations

1. datetime length from start/stop

   symbol	description	unit	variable name
   \(t_{e}\)	datetime stop	\(s\) since 2000-01-01	datetime_stop {time}
   \(t_{s}\)	datetime start	\(s\) since 2000-01-01	datetime_start {time}
   \(\Delta t\)	time duration	\(s\)	datetime_length {time}

   \[\Delta t = t_{s} - t_{e}\]