AT18 is January 18, 1974 LR01 is November 1, 1972 NPRST as the second letter indicates the year: 1970, 71, 72, 73, 74 The first letter is the month (but skip "I"). The 3rd and 4th digits are the day in that month. Larry Chace, Ithaca, NY I-5 and E15