A form with <input type="datetime-local"> posts "YYYY-MM-DDTHH:MM" with no offset. A server action doing new Date(raw).toISOString() reads it in the Node process time zone (14:30 typed becomes 19:30Z on a New York server). If the field was pre-filled from the stored UTC instant (value.slice(0,16)), every save shifts the value again and drops its seconds. Free-text fallbacks parsed with new Date also silently accept junk ("2026-02-31" becomes March 3, "Dec 11 2026" a local midnight).