Enough earrings to show the day of the month in hex (hex requires only 2 bits to show a number up to 255, more than enough to show 31 days of a month). The right ear, "the sixteens place" would have a 1 or 0 in it, and the left ear "the ones" would have 0-F (0-15). The 1st would be 01. The 13th would be 0d. The 25th would be 19. And so on. very simple math.