  We have developed a comprehensive methodology for calculating the
circumbinary HZ in planet-hosting P-type binary star systems. We present a
general formalism for determining the contribution of each star of the binary
to the total flux received at the top of the atmosphere of an Earth-like
planet, and use the Sun's HZ to calculate the locations of the inner and outer
boundaries of the HZ around a binary star system. We apply our calculations to
the Kepler's currently known circumbinary planetary system and show the
combined stellar flux that determines the boundaries of their HZs. We also show
that the HZ in P-type systems is dynamic and depending on the luminosity of the
binary stars, their spectral types, and the binary eccentricity, its boundaries
vary as the stars of the binary undergo their orbital motion. We present the
details of our calculations and discuss the implications of the results.
