Grade the answers as they were at the bell, and let them keep working

A backend engineer uses ?at_timestamp= to recover a key as it was before a bad write. A teacher uses it so nobody has to be locked out.

The quiz app stores each student's answers in the KV store: answers:ana, answers:bo, one key per student. The deadline is 16:00. Nothing happens at 16:00. No lockout, no freeze, no cron. Students keep editing if they want to; some of them learn the most in the ten minutes after.

Grading reads GET /api/v1/sqlite/kv/answers:ana?db=quiz.db&at_timestamp=1725292800. Whatever the key held at four o'clock is what gets marked. The 16:07 fix is visible in the history and worth nothing, and the student knows that going in.

Point-in-time recovery restores one key to any moment. Point-in-time grading reads one key at the moment that counted.