Invoice due date

I’m not sure if this is a bug or if it is by design, but I recently sent my first invoice and instead of a due date it simply says “in 21 days”. This may seem self explanatory but the fact that it doesn’t say “due” anywhere or have a specific date is not ideal. Any guidance on this is appreciated. The client I sent this to is a senior and doesn’t use her account so this invoice is the only information she has about payment details and I think it may be confusing to her. Thank you!