  A convective dynamo operating during the crystallization of white dwarfs is
one of the promising channels to produce their observed strong magnetic fields.
Although the magnitude of the fields generated by crystallization dynamos is
uncertain, their timing may serve as an orthogonal test of this channel's
contribution. The carbon-oxygen cores of $M\approx 0.5-1.0\,{\rm M}_\odot$
white dwarfs begin to crystallize at an age $t_{\rm cryst}\propto M^{-5/3}$,
but the magnetic field is initially trapped in the convection zone - deep
inside the CO core. Only once a mass of $m_{\rm cryst}$ has crystallized, the
convection zone approaches the white dwarf's helium layer, such that the
magnetic diffusion time through the envelope shortens sufficiently for the
field to break out to the surface, where it can be observed. This breakout time
is longer than $t_{\rm cryst}$ by a few Gyr, scaling as $t_{\rm break}\propto
t_{\rm cryst}f^{-1/2}$, where $f\equiv 1-m_{\rm cryst}/M$ depends on the white
dwarf's initial C/O profile before crystallization. The first appearance of
strong magnetic fields $B\gtrsim 1\textrm{ MG}$ in volume-limited samples
approximately coincides with our numerically computed $t_{\rm break}(M)$ -
potentially signalling crystallization dynamos as a dominant magnetization
channel. However, some observed magnetic white dwarfs are slightly younger,
challenging this scenario. The dependence of the breakout process on the white
dwarf's C/O profile implies that magnetism may probe the CO phase diagram, as
well as uncertainties during the core helium burning phase in the white dwarf's
progenitor, such as the $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ nuclear
reaction.
