Maybe @IntuitBettyJo can get the link fixed in my accepted answer at https://proconnect.intuit.com/community/lacerte-tax-discussions/discussion/how-do-i-get-the-date-to-... or look up the screen to do this in.
I know it is near the invoice input, enter as a negative to print on the 1040 and I am pretty sure it will pick it up on the invoice.
Screen 5.1. Client Letter - 3rd line down = date. Enter as a ' - ' (as @George4Tacks said).
It forces the date on the letter AND the invoice.
Code 4 on Screen 5.1 if you speak batch entry.