If the date or year of a publication is unknown, for example of a webpage, write “n.d.” for “no date”:
Royal Mail. (n.d.). 1st and 2nd class stamps. Retrieved January 21, 2025, from https://shop.royalmail.com/postage-and-packaging/first-and-second-class-stamps
If the publisher's name is missing, for example a self-published publication, omit this part of the reference:
Morin, D. J. (2024). Algebra: For the enthusiastic beginner: Volume 1.