  The Z Cam-type dwarf nova AT Cnc displays a classical nova (CN) shell,
demonstrating that mass transfer in cataclysmic binaries decreases
substantially after a CN eruption. The hibernation scenario of cataclysmic
binaries predicts such a decrease, on a timescale of a few centuries. In order
to measure the time since AT Cnc's last CN eruption, we have measured the
radial velocities of a hundred clumps in its ejecta with SITELLE, CFHT's
recently commissioned imaging Fourier transform spectrometer. These range from
-455 to +490 km/s. Coupled with the known distance to AT Cnc of 460 pc (Shara
2012), the size of AT Cnc's shell, and a simple model of nova ejecta
deceleration, we determine that the last CN eruption of this system occurred
$330_{-90}^{+135}$ years ago. This is the most rapid transition from a high
mass transfer rate, novalike variable to a low mass transfer rate, dwarf nova
yet measured, and in accord with the hibernation scenario of cataclysmic
binaries. We conclude by noting the similarity in deduced outburst date (within
a century of 1686 CE) of AT Cnc with a "guest star" reported in the
constellation Cancer by Korean observers in 1645 CE.
