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