Dates of birth are taken from narrations. Even there are different dates of the birth in Sunni books. Some Sunni scholars like ibn abdul Barr and al-Waaqidi . Al-Seira Al-Nabawiyyah by ibn Katheer said the Prophet was born on second of Rabaul Awwal ( not on the 12th)Ibn Abdul Barr in his book Al-Estee'aab. This has been mentioned in al-aseerah al-Nabawiyyah by ibn Katheer, v.1, p.199.
Some Sunni scholars said the Prophet was born on the eighth of Rabi,ul Awwal like Ibn Hazm and al-Zohri and al-Khawarizmi.
Other Sunni scholars said it was on the tenth of Rabi'ul Awwal like Ibn Dihyah and ibn Asaakir and al-Shi'bi.
Sime Sunni scholars narrated that the Prophet Muhammad (SAWA) was born in month of Ramadan or month of Safar. So,bayou see many different Sunni narrations in this matter.
We,v the followers of Ahlul Bayt (AS) have taken the most authentic narrations from Ahlul Bayt (AS) which is 17 Rabi'ul Awwal.