Imam al-Bukhari was among the rather severe scholars on the topic of al-Jarh wa-Ta'adeel (See What is Al Jarh Wa at-Ta'dil ( الجرح و التعديل )? and Is there a scale or classification for scholars and their qualification of hadith narrators?). He has compiled his own books of "Tareekh" (which are partly books on historical topics, but also and to large parts biographies) and weak narrators in which he has added essential information about hadith narrators. In fact most of the books of historians and hadith scholars on 'ilm ar-Rijal are a kind of mix of dates, biographical information based on reports and qualifications of people who met the person portrayed, typical narrator chains etc. other narrator chains of hadith etc. that the author has memorized from the portrayed person.
Note that basically the most important information for imam al-Bukhari is neither the birth nor death date, but the time span in which the narrator might have studied under a specific teacher, so the essential information is who were his teachers and/or who were his students. Therefore if these data and dates are not available narrator chains and further information are of big help. As a person who was studying in al-Hijaaz could hardly be in Khorassan at the same time.
Books of imam al-Bukhari on the topic of al-Jarh wa at-Ta'adel or 'ilm ar-Rijal and some of their content
In fact he had compiled the following books (See also on wikipedia and the Arabic wikipedia page):
- At-Tareekh al-Kabir التاريخ الكبير (literally: The great history): The large compendium (This is published).
Bukhari authored this work when he was 18 years of age. He compiled the names and biographies of many of those who narrated hadith from the time of the Companions until his time. The total number of biographies included is close to 40 thousand, including both men and women, reliable and unreliable narrators. However, al-Hakim enumerated the weak narrators of that total number and they were not more than 126 individuals. (Source wikipedia)
From a first sight this book looks like a book of musnad narrations. However it might be very cryptic to read for a layman here just a random example of two entries:
In the following I'm translating from Arabic language, as these translations are of my own take them carefully
868 - مُحَمَّد رأى ابْن الزبير روى عَنْهُ ابنه يونس. (Source)
Muhammad saw ibn al-Zubayr his son Yunus narrated on his authority.
869 - مُحَمَّد قَالَ لِي إِسْحَاقُ حَدَّثَنَا حِبَّانُ قَالَ حَدَّثَنَا هَمَّامٌ عَنْ قَتَادَةَ قَالَ حَدَّثَنِي مُحَمَّدٌ وَعَوْنٌ أَنَّهُمَا قَالَا لِسَعِيدِ بْنِ الْمُسَيِّبِ أَنَّ عَطَاءً الْخُرَاسَانِيَّ حَدَّثَنَا عَنْكَ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَتَاهُ رَجُلٌ وَقَعَ بِأَهْلِهِ فِي رَمَضَانَ فَأَمَرَهُ أَنْ يَعْتِقَ رَقَبَةً، فَقَالَ كَذِبٌ مَا حَدَّثْتُهُ إِنَّمَا قَالَ تَصَدَّقْ تَصَدَّقْ.
Muhammed said Ishaaq informed us Hibbaan said we were informed by Hammam from Qatadah, who said Muhammed and 'Aoun infromed me and that they said to Sa'id ibn al-Musayyib that 'Ataa' al-Khorasani informed us from you that a man who had intercourse with his wife in Ramadan came to the Prophet, Allah's prayers and blessings be upon and he ( the prophet) ordered him to free a slave, he ( Sa'id ibn al-Mussayib) said, he lied but what I told him, was that he ( the prophet) said give charity, give charity.
(See a related hadith in Sahih Muslim)
And here a random example of an entry including a date:
890 - إِبْرَاهِيم بْن بشار أَبُو إِسْحَاق الرمادي سمع ابن عيينة، يهم في الشئ بعد الشئ (يقال مات بالبصرة سنة أربع وعشرين ومائتين ) .
إِبْرَاهِيم أَبُو بكير عَنْ رجل من الأنصار عَنْ أَبِي هُرَيْرَةَ روى عَنْهُ مُحَمَّد صاحب الساج، مرسل، حدثني أزهر قَالَ حَدَّثَنَا عَبْد الوهاب قَالَ حَدَّثَنَا هشام عَنْ إِبْرَاهِيم بْن أَبِي بكير عَنْ علي بْن زيد أن كلاب بن امية استعمل على الابلة. (Source)
Ibraheem ibn Bashar abu Isahaaq ar-Ramaady who studied under ibn 'Uyyaynah, he mistakes in some details (literally: in a thing after a thing) - it is said that he died in Basra 224 a.H.).
Ibraheem abu Bukayr ( This is his Kunya) on the authority of a man from the Ansar, on the authority of abu Hurairah. Muhammad (ibn abi Sahl) Sahib as-Saj, narrated from him as mursal, Azhar told me abd al-Wahhab informed us, and said, Hisham on the authority of Ibraheem ibn abi Bukayr, on the authority of 'Ali ibn Zayd, that Kilaab ibn Umayyah was governor on al-Iblah.
- At-Tarikh al-Awasat التاريخ الأوسط: The medium compendium (Has wrongly been published as the small compendium)
- At-Tareekh as-Saghir التاريخ الصغير: The small compendium (Is lost and focused on Sahabah).
- Al-Kuna الكُنى: a book on patronymics. Narrators who have been known by their Kunya while their real name was unknown.
- A-Do'afaa' as-Saghir الضعفاء الصغير: The small book on weak narrators. Here al-Bukhari would add qualifications of earlier scholars if available. Here again some random quotes:
43 - باذام أَبُو صَالح مولى أم هَانِئ الهاشمية أُخْت عَليّ بن أبي طَالب كُوفِي قَالَ بن سِنَان ترك بن مهْدي حَدِيث أبي صَالح حَدثنَا مُحَمَّد ثَنَا أَحْمد بن عبيد عَن مُحَمَّد بن قيس عَن حبيب بن أبي ثَابت قَالَ كُنَّا نسمي أَبَا صَالح باذام دروع زن (Source)
Baadham abu Salih Mawla Umm Hani' al-Hashimiyya sister of 'Ali ibn Abi Talib, a Kofi. Ibn Sinan said ibn Mahdy left the hadith of abu Salih. Muhammad ibn 'Ubayd informed us on the authority of Muhammad ibn Qais on the authority of Habib ibn abi Thabit, who said that we used to call abu Salih Baadham duru' zin (In a different narration of the book: Baadham Durughazin and it was also said Baadhaan باذام. دروغزن، ويقال: باذان.).
49 - جَابر بن يزِيد الْجعْفِيّ تَركه يحيى بن مهْدي قَالَ أَبُو نعيم مَاتَ سنة ثَمَان وَعشْرين وَمِائَة يروي عَن الْقَاسِم وَعَطَاء وَالشعْبِيّ قَالَ علي أرَاهُ أَبُو يزِيد قَالَ بَيَان سَمِعت يحيى بن سعيد يَقُول تركنَا جَابِرا قبل أَن يقدم علينا الثَّوْريّ وَقَالَ أَبُو سعيد الْحداد سَمِعت يحيى بن سعيد عَن إِسْمَاعِيل بن أبي خَالِد قَالَ قَالَ الشّعبِيّ يَا جَابر لَا تَمُوت حَتَّى تكذب على رَسُول الله صلى الله عَلَيْهِ وَسلم قَالَ إِسْمَاعِيل فَمَا مَضَت الْأَيَّام والليالي حَتَّى اتهمَ بِالْكَذِبِ (Source)
Jaaber ibn Yazid al-Ja'fy left (regraded as matrook) by Yahya bin Mahdi. Abu Nu'aym said he died the year 128 (a.H.). He narrates from al-Qassim, 'Ata' and a-Sha'aby. 'Ali said I regard him ( referring to) as abu Yazid. Bayan said I heard Yahya ibn Sa'id saying we left Jaaber before a-Thwary came to us . And Abu Sa'eed al-Haddaad said I heard Yahya bin Sa'id from Ismail ibn Abi Khalid said: Al-Sha'bi said, O Jabir, do not die until you lie to the Messenger of Allah, may Allah’s prayers and blessings be upon him. Ismail added, so the days and nights went by and he was accused to be a liar.
- A-Do'afaa' al-Kabir الضعفاء الكبير: The large book on weak narrators (which contains more details on each presented narrator compared to the small book).
Based on this database and knowledge imam al-Bukhari gathred over years of studies he was able to qualify narrators and he had reports of people and their dates of birth, reports of when a person was were and at least approximate dates of studying with a certain teacher at hand.
The methodology is widely explained in the following posts: