E14. Two-block calendar

A calendar consists of two cubes of the same size, about 2 inches on a side. Each cube contains a single digit. When placed together, the front faces of the two cubes display the day of the month, from 01 to 31. Note that single digit days must be displayed as two digits, with a leading 0. Describe what digits to place on each face of the cubes for this to work.