In Keyboard Maestro, perform date math with predefined date variable?

Hello! I’m having trouble finding the answer to this elsewhere.

All the date math information I can find for KM is generally based on the current date (i.e. 3 days before today). Is there a way I can add a variable with a predefined date? Specifically the macro asks the user to enter the relevant date (say, March 20th) and I would like to perform date math later in the macro (3 days prior to March 20th, giving March 17th).

While less important, is there a way to have KM fill in a date based on a day of the week? Specifically having it default to last Friday’s date?

Thanks so much!