Summary

• expand the shared date library into the single API for display formats, parsing, relative time, elapsed durations, comparisons, expiry checks, and date arithmetic
• replace component-local formatters and duplicated duration helpers across settings, logs, metrics, backups, builds, and rollouts
• standardize server-side expiry, threshold, retention, scheduling, and metrics-window calculations
• add a hydration-safe local date component so server-rendered timestamps switch to the viewer timezone without React hydration mismatches
• validate agent timestamps and backup retention configuration before date arithmetic

Why

Datetime behavior was split across local helpers, direct Intl and Date calls, and repeated timestamp arithmetic. This produced inconsistent formats, timezone-sensitive server output, duplicated logic, and edge cases around invalid or out-of-range timestamps.

This change keeps persistence timestamps and protocol ISO serialization explicit while routing application-level date behavior through one shared module.

Basecamp: https://app.basecamp.com/5890597/buckets/47439993/todos/10081400176

Impact

• UI dates use the shared en-US, 24-hour format contract in the viewer local timezone
• server-generated alert timestamps use explicit UTC
• expiry checks fail closed on invalid data
• elapsed and UTC calendar operations are named and tested separately
• no new runtime dependencies were added

Review

• completed a full self-review and addressed naming, fail-closed expiry, and rendering issues
• completed Claude Opus smart review and follow-up review
• fixed the unauthenticated agent timestamp overflow path, date arithmetic overflow validation, hydration-safe rendering, configuration bounds, and review follow-ups

Validation

• Biome check on all 33 changed files
• TypeScript: tsc --noEmit
• Vitest: 17 files, 81 tests
• Next.js 16 production build