Underscore as a separator, YYYY_MM_DD:

  1970_01_01
  2024_06_09
  2024_12_31
  16383_12_31

Treated as base-10 number literal in some programming languages.

5 bits for DD [0, 31] + 4 bits for MM [0, 15] + 14 or more bits for year [0, 16_383] = 23 bits.