  We present stellar rotation periods for late K- and early M-dwarf members of
the 4 Gyr old open cluster M67 as calibrators for gyrochronology and tests of
stellar spin-down models. Using Gaia EDR3 astrometry for cluster membership and
Pan-STARRS (PS1) photometry for binary identification, we build this set of
rotation periods from a campaign of monitoring M67 with the
Canada-France-Hawaii Telescope's MegaPrime wide field imager. We identify 1807
members of M67, of which 294 are candidate single members with significant
rotation period detections. Moreover, we fit a polynomial to the period versus
color-derived effective temperature sequence observed in our data. We find that
the rotation of very cool dwarfs can be explained by a simple solid-body
spin-down between 2.7 and 4 Gyr. We compare this rotational sequence to the
predictions of gyrochronological models and find that the best match is
Skumanich-like spin-down, P_rot \propto t^0.62, applied to the sequence of
Ruprecht 147. This suggests that, for spectral types K7-M0 with near-solar
metallicity, once a star resumes spinning down, a simple Skumanich-like is
sufficient to describe their rotation evolution, at least through the age of
M67. Additionally, for stars in the range M1-M3, our data show that spin-down
must have resumed prior to the age of M67, in conflict with predictions of the
latest spin-down models.
