datetime derivations

1. datetime from start/stop

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

   \[t = \frac{t_{s} - t_{e}}{2}\]