The oldies and classic rock formats have a strong niche market, but as the audience becomes older the station becomes less attractive to advertisers. Advertisers perceive older listeners as set in their brand choices and not as responsive to advertising as younger, more impulsive listeners. Oldies stations must occasionally change to more youthful music formats; as a result, the definition of what constitutes an "oldies" station has gradually changed over the years (and the phrase "oldies" itself is falling out of use except for the stations that still regularly play music from the 1960s and earlier). This is why many oldies stations, like WCBS-FM in New York City and WJMK in Chicago, have switched over to the younger-oriented Jack FM format in recent years—although WCBS-FM adopted a Classic Hits format on July 12, 2007, and the "Jack FM" format was moved to its HD2 subchannel. Unlike WCBS-FM's pre-JACK format which was centered on the 1955-1979 era, the post-JACK station was based on the 1964-1989 era because of the aging listener demographics of the original format.