Validates the month field is 01–12 and the day field is 01–31, rather than accepting any two digits (which would let 2026-13-99 through). This is a date-only pattern — for full ISO 8601 timestamps with time and timezone, that's a different, more complex pattern.
What it doesn't do: reject invalid combinations like February 30th or April 31st — checking the day range against the specific month (and leap years) requires more than a regex can reasonably express; validate that with actual date-parsing logic after the format check passes.