New Post: unique properties for products

You could add a single product with multiple models, one for each number of days... e.g. Days: 1,2,3,4,5,10,20 or whatever is typical.
Each model gets its own price.
Then add a custom field with a date picker to select the start date.
The end date would be assumed to be the start date plus the number of days of the model chosen.

Regards
Rob