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A key event in the history of the Milky Way is the formation of the bar. Thisevent affects the subsequent structural and dynamical evolution of the entireGalaxy. When the bar formed, gas was likely rapidly funnelled to the centre ofthe Galaxy settling in a star-forming nuclear disc. The Milky Way bar formationcan then be dated by considering the oldest stars in the formed nuclear stellardisc. In this highly obscured and crowded region, reliable age tracers arelimited, but bright, high-amplitude Mira variables make useful age indicatorsas they follow a period--age relation. We fit dynamical models to the propermotions of a sample of Mira variables in the Milky Way's nuclear stellar discregion. Weak evidence for inside-out growth and both radial and verticaldynamical heating with time of the nuclear stellar disc is presented suggestingthe nuclear stellar disc is dynamically well-mixed. Furthermore, for Miravariables around a $\sim350$ day period, there is a clear transition fromnuclear stellar disc-dominated kinematics to background bar-bulge-dominatedkinematics. Using a Mira variable period-age relation calibrated in the solarneighbourhood, this suggests the nuclear stellar disc formed in a significantburst in star formation $(8\pm 1)\,\mathrm{Gyr}$ ago, although the data arealso weakly consistent with a more gradual formation of the nuclear stellardisc at even earlier epochs. This implies a relatively early formation time forthe Milky Way bar ($\gtrsim8\,\mathrm{Gyr}$), which has implications for thegrowth and state of the young Milky Way and its subsequent history.

The epoch of the Milky Way's bar formation: dynamical modelling of Mira variables in the nuclear stellar disc