My name is Rajagopal 70 years old, I knows basic electronics but no experience in current programming. I have purchased a few 7 segment 4 digit with AT89C52, RTC DS1307, 74LS595 (4nos), and ULN 2003 (4nos) 12hrs format and program preloaded assembled clock PCBs. I did 12” , and 15” height and 4’ long LED display with driver circuits, this clocks are working since 2- 3 years, with Accuracy and good condition. I made a another display for another clock board, but my soldering mistake it was not work , found the fault and modified it and found working 3 digit only, the 4th digit of 10hrs digit not work, I mean, run from 0 : 0 0 to 9 : 5 9 after 9 : 5 9 it is not going to 1 0 : 0 0 instead 1 0 : 0 0 it is going again 0 : 0 0 (three digits zeros) and run up to 9 : 5 9 then again going 0 : 0 0 and run up to 3 : 5 9, after 3 : 5 9 it is going to 0 : 0 0 it mean, it is working as 10hrs + 10hrs + 4hrs for 24hrs, instead 12hrs format (12hrs + 12hrs = 24hrs). Removed DS1307 ic and replaced a new DS11307, but working as above. Not only one, few new DS1307 ics replaced but same as above. Now I doubt in 89c52, what happened changed the program code in the 89C52 it self, may it will possible?. Circuit and program are designed with AT89C52 for 12 hrs format earlier it was worked I assembled two clocks still it is working, some unknown reason RTC DS1307 is changed to 24 hrs format why?, and how to get it back to 12hrs format, may any one help me, please.