I’m not sure if this is a meme post or you’re sincere but I’ll assume the latter and respond. Hopefully the mods don’t delete my comment for not quoting primary or secondary documents.

We don’t necessarily need “technology” to know what date something happened hundreds of years ago. People hundreds of years ago had writing and complex calendar systems. Also important to remember that “hundreds of years ago” could mean the 1820s at this point. At this point newspapers existed all over the world and had existed for hundreds of years. I used to read diaries from about 100 years ago as part of my undergrad research. The housewife’s diary I focused on had date and time of day written for every entry: “June 12th, Evening” etc.

For older events or less recorded events historians don’t always have an exact date but they can make best guesses based on certain events. If there is a known event that can be dated based on mentions of the calendar date they can use other primary sources. If my diarist wrote that “10 days ago” there had been a murder in her town on June 12th one could assume the date of that murder was the 2nd of June.

If you mean thousands of years ago and not hundreds of years ago, it’s often a mix of the above methods as there are still many written records. Also in certain cases astrological events can be used to get an exact date. Because astrological events can be calculated to an exact date with perfect precision if someone records an eclipse that date can be used to figure out the dates of other events.