# 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}

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