"...
the date at the top of the letter is the date on which I purchased the software
"
I have to assume you are using TurboTax Business (not Home & Business) because that is the only TurboTax version that is capable of generating K-1s for distribution to beneficiaries.
The program uses the current date for the date that appears on the Beneficiary Letter. I just started my copy of TurboTax Business and the date is April 2, 2019 (see screenshot). The date cannot be edited from within the program, but can easily be edited with PDF software that has editing functionality.
If your letters have the date the software was purchased, something is seriously wrong; the date should be the current date which, presumably, is derived from your computer. Thus, you should check your PC's date/time and also check to ensure the program is properly installed. Beyond that, you can contact support (link below) for an operational issue of this nature.
https://ttlc.intuit.com/questions/1899263
