Function marker 

pub fn marker(
    until: Option<Timestamp>,
    wall_now: Timestamp,
    boottime_now_s: u64,
) -> String

The marker’s content: boot, or the expiry in Unix seconds and the same deadline on the boot clock, boottime_now + (until - wall_now), both truncated to seconds. A deadline already past saturates to boottime_now or below: expired.