White dwarfs are stellar embers that simply cool down for the rest of time, eventually freezing into a solid state. This predictable evolution makes them precise cosmic clocks; they have been used for decades to measure the ages of stellar populations. But data from the Gaia space observatory is now calling into question the accuracy of this age dating technique. The cooling process appears to be much more delayed by the onset of crystallization than predicted by current models. I will present my recent work on the physics of white dwarf crystallization. In particular, I will explain how the fractionation of 22Ne can eliminate the current disagreement between models and observations. I will also discuss our current work on 3D hydrodynamics simulations of red giant stars with the aim of better constraining the interior composition of white dwarfs, a key uncertainty for white dwarf age dating.