21 day return policy

I am wondering we cancelled our direct tv service and our service is good until january 9th, why would we be expected to return the equiptment before then?? My wife called on december 9th to cancel and they told her she has 21 days from that call to retturn the boxes, if we paid till january 9th why would we be expected to return the boxes before tge 9th?? You would the the 21 days starts from the last day of you service??