  Age is one of the most fundamental parameters of stars, yet it is one of the
hardest to determine as it requires modelling various aspects of stellar
formation and evolution. When we compare the ages derived from isochronal and
dynamical traceback methods for six young stellar associations, we find a
systematic discrepancy. Specifically, dynamical traceback ages are consistently
younger by an average of $\langle\Delta_{\rm Age}\rangle = 5.5 \pm 1.1$ Myr. We
rule out measurement errors as the cause of the age mismatch and propose that
$\Delta_{\rm Age}$ indicates the time a young star remains bound to its
parental cloud before moving away from its siblings. In this framework, the
dynamical traceback "clock" starts when a stellar cluster or association begins
to expand after expelling most of the gas, while the isochronal "clock" starts
earlier when most stars form. The difference between these two age-dating
techniques is a powerful tool to constraint evolutionary models, as isochronal
ages cannot be younger than dynamical traceback ages. Measuring the
$\Delta_{\rm Age}$ accurately and understanding its variations across different
environments will provide further information on the impact of local conditions
and stellar feedback on the formation and dispersal of stellar clusters.
