A class action was filed yesterday in U.S. District Court for the District of Columbia accusing Groupon Inc. of placing unlawful expiration dates on its gift certificates.

The Web site, which has headquarters in Chicago and an office in Palo Alto, Calif., works with businesses in cities across the United States and around the world to offer consumers discounted rates on goods and services, from restaurant meals and flowers to hot air balloon rides. The company sells these “Daily Deals” online.