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Knowing the ages of low-mass stars is key to constraining the habitability oftheir planets. FGKM stars spin down due to loss of angular momentum viamagnetized winds and rotation-period and magnetic-activity measurements arewindows into what should be the most reliable clocks in these stars. Howeverobservations of open clusters whose known ages make them ideal testbeds for theage-rotation-activity relation (ARAR) show some single >1 Gyr old stars notbehaving their age with rotation remaining unexpectedly rapid. X-rays are themost direct way of testing explanations for this stalling in spin-down. Wepropose to turn Ruprecht 147 at 300 pc the closest old cluster into a newbenchmark for the ARAR by measuring coronal activity at 2.7 Gyr when K and M stars are acting half their age. [truncated! Please see actual data for full text]