I think this will work but have not tested it.
Create your param and make your list of options.
If you use text options then try this replacing the text and numbers as needed:
table.date in dateadd('d',-
(if {?param}="1 week" then 7 else if {?param}="2 week" then 14 else if {?param}="3 week" then 21 ... etc.)
,currentdate) to currentdate
If you use number text in your param try:
table.date in dateadd('d',-(totext({?Param},0,'')),currentdate) to currentdate
If you use numbers in your param try:
table.date in dateadd('d',-({?Param}),currentdate) to currentdate