First, find the days when they will tell the truth. January has 31 days:
Prime: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31
Third: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30
The dates on which they will both tell the truth are the 3rd, so there is only 1 date that works.