0011. Alert when strictly fewer than N whole days remain

• Status: Accepted
• Date: 2026-07-11

Context

The expiry-threshold fire/no-fire equivalence quietly depends on both halves working together — the strict < in the decision switch and the truncate-toward-zero in daysUntil. An innocent <=, a math.Round, or a well-meaning “fix” of the truncation asymmetry would shift the boundary by up to a full day, uncaught.

Decision

Pin the contract: warning_days: N fires when strictly fewer than N days remain (critical_days likewise). Whole days are counted truncated toward zero. For integer N this makes floor(remaining) < N ⟺ remaining < N × 24h exactly, so the alert decision agrees with a Prometheus line at N * 86400 over certel_cert_expiry_timestamp_seconds - time() (up to probe staleness).

Consequences

• Establishes a must-not-regress invariant tying the code (days < *t.WarningDays plus truncate-toward-zero daysUntil) to the Prometheus threshold equivalence — the reason a dashboard rule and the alert never disagree by a day.
• At the boundary: N days − 1s → warning; exactly N days and N days + 1s → ok; the same pair around critical_days; the already-expired side (read as day 0) still fires.
• Result.DaysLeft is computed once per probe from the injected clock (p.Now()), never from a second wall-clock read — so logs, metrics and the threshold decision see the same number, the property the boundary tests rely on.

References

• internal/probe/probe.go (daysUntil, threshold switch), internal/probe/result.go (DaysLeft).
• “Check statuses” section in the README .