score:3
As always when an electronic visa application rejects correct answers for technical reasons, the goal is to be clear. Give answers that are as close to the truth as the system will let you, and then enclose an explicit explanation (as a separate sheet, or in an appropriate free-text field) that explains clearly why you've needed to give wrong answers and what the right answer is. That way it won't look like you're trying to deceive or hide anything.