datetime start derivations

1. datetime start from datetime and stop

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

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