There is a difference of opinion, some of Ulema says that RasooluLLAH (PBUH) was born on 7th of Rabi-ul-Awwal, some say on 9th, but most of them have opinion that he was born on 12th of Rabi-ul-Awwal.
RasooluLLAH left this world before He (PBUH) was 63. About a week earlier. Some say week some say 10 days. some say 8 days. etc. But positively, he was about to be 63 (in a few days) when he left.
Now, if he was born on 7th, then He left in the end of Safar or beginning of Rabi-ul-Awwal, any day before 7th. Now if someone says that RasooluLLAH (PBUH) left on 12th of Rabi-ulAwwal, then it doesn't make sense at all. Because that suggests that He (PBUH) was born on 18th etc. I'm sure no-one will agree on that. So apparently this "leaving on 12th of Rabi-ul-Awwal" is a newly crafted thing by the sect worshippers or conspiracy theorist enemies of Islam who are trying to keep Muslims confused.