well a lot of the tablets actually have the date printed on them. of course its not like "2/11/09" or something. A lot use astrological methods which are still know today. Also, writing is found within the tombs of kings and such. Radiocarbon dating on the kings can help date those. To be truthful, even stuff written a milena before Jesus was supposedly around is pretty darn clear. Very early chinese character writing was found on turtle shells, radiocarbon dating to around 6500BC. On cuneiform tablets, the name of the scribe, date, and the city were it was written is inscribed on the tablet. Hieroglyphs are from about 3000 BC too and are also detailed. We know what they say because translations of Egyptian hieroglyphs into Greek have been found. The spoken language of ancient Egypt is very similar to Coptic, which is widely known.