It’s 2020 - don’t shorten it to 20!
Matter of fact. When you enter into a contract, it will almost certainly have a start and/or completion date. It may seem a rather odd thing to say, but now that it’s 2020 you should write the year of the date in full, i.e. 2020 or 2021, as opposed to 20. Why is this? The first reason is that it will prevent someone altering the document. Another number could easily be added to the end of 20, e.g. to make it, say, 2019 or 2018. In some circumstances, this will have serious implications.
For clarity. Also, if you write the year in full, you will have clarity on the year a document was created; there won’t be any queries further down the line about the year it came into existence. Electronic documents will, of course, have other information hidden in the file but that won’t apply to any hard copy documents you use.