Converts a date-only string (e.g. "2026-03-05") to the end of that day in UTC. Returns an ISO datetime string like "2026-03-05T23:59:59.999Z".
Converts a date-only string (e.g. "2026-03-05") to the end of that day in UTC. Returns an ISO datetime string like "2026-03-05T23:59:59.999Z".