How far can we see back in time in high-energy collisions using charm hadrons?

We use open charm production to estimate how far we can see back in time in high-energy hadron-hadron collisions. We analyze the transverse momentum distributions of the identified D mesons from pp, p--Pb and A--A collisions at the ALICE and STAR experiments covering the energy range from $\sqrt{s_{\rm NN}} = 200$~GeV up to 7~TeV. Within a non-extensive statistical framework, the common Tsallis parameters for D mesons represent higher temperature and more degrees of freedom than that of light-flavour hadrons. Assuming Bjorken-expansion, the production of D mesons corresponds to a significantly earlier proper time, $\tau_{\rm D} = (0.18 \pm 0.06) \tau_{\rm LF}$.