In many countries you can have a statement you wrote signed, dated and stamped by the state authorities that verifies not the truth of the content, but that _you_ wrote it (and the specific time). You need to fill a form, take it to a state office, and present your ID to them for them to stamp and approve it.
In the US I find that this is called a "statutory declaration":
A statutory declaration is a document that formalizes matters to be made known publicly. It is a solemn statement made by plaintiff or witnesses instead of the oath, but equally binding.
Statutory declaration is a legal document based on statute law as to their format and content requirements. Statutory declarations are of the same force and effect as if sworn under oath or affirmation.