How do historians date ancient documents?

One way historians date documents is through a method called palaeography which studies scribal habits etc. and through the analysis of distinctive writing materials, and handwriting style allows palaeographers to date letters to within centuries and decades.

It is similar to somebody 5000 years from now looking back and being able to see the transition from handwriting to typing in our documents. As well they could see the development of the use of acronyms (i.e. lol) for instant message or text message users, and based on other known dates, such as the invention of the computer, and work the approximate dates of letters and documents out from there. Of course this may be unnecessary in the future, but you get the point.

Documents can be given very broad dates by the places they are found, such as important buildings, or cities, and based on their attestation in other documents which have clear dates. Overall its not exact, and we have to give +/- room, but we can get very close to the date of composition, mainly through these methods.