message: Add instances and tweak publicity for types
diff: use std::time::{Duration, SystemTime};
  /// Represent a timeline with a specific start point rooted on earth time.
  +#[derive(Debug, Clone)]
  pub struct Timeline(pub(crate) SystemTime);
  /// Represent an offset in time units in the timeline
  +#[derive(Debug, Clone)]
  pub struct TimeOffset(pub(crate) Duration);
  impl From<SystemTime> for Timeline {
  