I am being sued for an unpaid credit card debt. I just received the summons today, but the statue of limations has expired. What should I do?

You have astutely investigated the statute of limitations relative to open accounts, which credit cards are presumed to be.  NRS 11.190(2)(a) provides that an action on any open account may only be commenced "[w]ithin 4 years. . . on an open account for goods, wares and merchandise sold and delivered."  The time in NRS 11.190 shall be deemed to date from the last transaction or the last item charged or last credit given; if payments have been made on the account, "the limitation shall commence from the time the last payment was made." NRS 11.200.  Statutes of limitation can be extended or stayed if "contained in some writing signed by the party to be charged thereby." NRS 11.390.  Conversely the statute of limitations for an obligation or liability founded in writing is 6 years.  NRS 11.190(2)(a). There is an exception to the open account analysis which the Nevada Supreme Court opaquely created 40 years ago however in El Ranco, Inc. v. New York Meat and Provision Co, 88 Nev. Ill, 115 (1972).  In that case, the Court looked at sales on an otherwise open account in which each delivery was memorialized by sales slips which acknowledging receipts, the Nevada Supreme Court stated that the action was upon obligation founded upon instrument in writing to which 6-year statute of limitations was applicable because when buyer signed sales slips, sales became consummated and the sales slips showed upon their faces both liability to pay and obligation to pay.  Therefore the existence of sales slips might be argued to create a claim not on the open account but on the underlying transactions.