when cron last ran, or null if it never has. NEVER RUN IS NOT DUE: the first alarm on a fresh site is the busiest one it will ever have -- migration, the first fills, the first render -- and adding an interpreter unit to it buys nothing, because a site with no content has nothing to schedule, index or expire. The caller stamps the clock instead, so the first pass lands one interval after boot.
Whether enough time has passed since the last firing.