Passport application - Grandparents details

[Jk2007]

If both parents were born outside UK, the application is asking grandparents name, town and country of birth, and date of marriage.

I can get only name, town and country of birth, but could not get the date of marriage as it not known. So what should I do? Should I write date of marriage unknown?

[Casa]

This is intended for those who are applying through parentage.
Write 'unknown'.