On the basis of the time it takes for a generation to mature, U.S. authors William Strauss and Neil Howe define Generation X as those born between 1961 and 1981 in their 1991 book titled "Generations", and differentiate the cohort into an early and late wave. Jeff Gordinier, in his 2008 book "X Saves the World", also has a wider definition to include those born between 1961 and 1977 but possibly as late as 1980. George Masnick of the Harvard Joint Center for Housing Studies puts this generation in the time-frame of 1965 to 1984, in order to satisfy the premise that boomers, Xers, and millennials "cover equal 20-year age spans". In 2004, journalist J. Markert also acknowledged the 20-year increments but goes one step further and subdivides the generation into two 10-year cohorts with early and later members of the generation. The first begins in 1966 and ends in 1975 and the second begins in 1976 and ends in 1985; this thinking is applied to each generation (Silent, boomers, Gen X, millennials, etc.).