It has to be done in INK (can you can use erasable ink pens and get away with it
). I've used flexible passes in Switzerland, when the conductor did his ticket check, he used his official dating stamp and put the mark over the entire box (containing the date)--so it would be hard to erase and write over the stamp. In France they don't place the stamp when they check tickets. One date saver with flexpasses, if you are taking an overnight train that departs after 7 pm, you list the
next day as your date.
You must also have your date written in BEFORE you board the train.. or at least when you're already in your seat. Trying to avoid getting checked or simply saying you 'forgot' means that you're trying to travel without a valid ticket / pass and you could be liable to a fine.
If you've entered a wrong date down, you cannot simply cross it out and put in a new date. It is voided, which means you LOST that day and you must put in a new date in a new box.