When doing research on ancient history and going to websites like Wikipedia, it will tell you the date that an event occurred, but it isn't usually clear why a certain event was dated that way.
For example, the Wikipedia article for the Great Fire of Rome states: " The Great Fire of Rome was an urban fire in July of the year AD 64".
How did they actually get this date? Are there any good techniques in finding out why something was dated the way it was?
This is not a problem most of us worry a great deal about, and honestly the answer is nowhere near as exciting as you might think it is. For a start, there is a post in the AskHistorians FAQ by u/sunagainstgold that details how the modern BC/AD dating system was created, and how it has been matched up with other dating systems.
In terms of dating events in Roman history, the Romans themselves kept relatively good records. Every year was known by the names of the consuls who served, and those records were displayed in public. The fasti capitolini is the best preserved list of consuls, listing the names of magistrates from the fifth century BC up to the year before the death of Augustus in 14 AD. We are also helped by the 'annalistic' tradition of historical writing that was popular in Rome, which was organised on a year-by-year basis. These various texts aren't perfect, and even the Romans themselves had problems with dating their own history. The biggest issue is the fact that four pairs of consuls seem to be missing from some time in the fourth century BC. Various solutions were proposed in Antiquity, but none are totally satisfactory - you can read about the whole thing here if you like, although I must admit that it makes my head hurt just thinking about it.
Anyway, Tacitus's Annals was influenced by the annalist tradition I mentioned above (although the work is not strictly 'annalist' because he doesn't simply record the events that happen in each year, but gives his interpretation and opinion), and it's in this work that the Great Fire of AD 64 is recorded. Tacitus says that the fire happened:
C. Laecanio M. Licinio consulibus
In the consulate of Gaius Laecanius and Marcus Licinius
We know Tacitus wasn't totally making this up because there's an inscription attesting their consulship. That's not to say there are no problems with other dates given in Tacitus, and there are scholarly arguments about dating various historical events, from an earthquake in Campania (either AD 62 or 63, or both) to Domitian's war against the Chatti (82-83 or 83-85?). Problems usually arise when we have multiple accounts of the same event from different authors, but who give slightly different evidence for the dating. As I alluded to at the beginning, the way these dispute are settled are excruciatingly boring, and very rarely make the headlines.