I recently received this message about the importance of writing the year 2020 in full, and I thought it might be useful to pass it on here :
Always write the year in full, i.e. 23/01/2020 and not 23/01/20 because in the second case, an ill-intentioned person could add 2 figures (2019, 2018 etc) and invalidate your document. They could also pre-date your document or authorisation.