I do not have crystal in front of me so cannot test and apologize for any errors or misunderstandings.
What i get from your post is that you have a parameter that returns a user entered date value.
If that value is before December 7 2012, you want the date to default to December 7, 2012.
create two formulas, one for the start date and one for the end date and use them in your select expert:
//{@OnDateStart}
IF {%DateStart} < Dateserial(2012,12,07) then Dateserial(2012,12,07) else {%DateStart}
//{@OnDateEnd}
IF {%DateEnd} < Dateserial(2012,12,07) then Dateserial(2012,12,07) else {%DateEnd}