  The epoch in which galactic discs settle is a major benchmark to test models
of galaxy formation and evolution but is as yet largely unknown. Once discs
settle and become self-gravitating enough, stellar bars are able to form;
therefore, determining the ages of bars can shed light on the epoch of disc
settling, and on the onset of secular evolution. Nevertheless, until now,
timing when the bar formed has proven challenging. In this work, we present a
new methodology for obtaining the bar age, using the star formation history of
nuclear discs. Nuclear discs are rotation-supported structures, built by gas
pushed to the centre via bar-induced torques, and their formation is thus
coincident with bar formation. In particular, we use integral field
spectroscopic (IFS) data from the TIMER survey to disentangle the star
formation history of the nuclear disc from that of the underlying main disc,
which enables us to more accurately determine when the nuclear disc forms. We
demonstrate the methodology on the galaxy NGC 1433 -- which we find to host an
old bar that is $8.0^{+1.6}_{-1.1}\rm{(sys)}^{+0.2}_{-0.5}\rm{(stat)}$ Gyr old
-- and describe a number of tests carried out on both the observational data
and numerical simulations. In addition, we present evidence that the nuclear
disc of NGC 1433 grows in accordance with an inside-out formation scenario.
This methodology is applicable to high-resolution IFS data of barred galaxies
with nuclear discs, making it ideally suited for the TIMER survey sample. In
the future we will thus be able to determine the bar age for a large sample of
galaxies, shedding light on the epoch of disc settling and bar formation.
