  The discovery with the {\it Herschel Space Observatory} of bright far
infrared and submm emission from the ejecta of the core collapse supernova
SN\,1987A has been interpreted as indicating the presence of some
0.4--0.7\,M$_\odot$ of dust. We have constructed radiative transfer models of
the ejecta to fit optical to far-infrared observations from the literature at
epochs between 615 days and 24 years after the explosion, to determine when and
where this unexpectedly large amount of dust formed.
  We find that the observations by day 1153 are consistent with the presence of
3$\times$10$^{-3}$M$_\odot$ of dust. Although this is a larger amount than has
previously been considered possible at this epoch, it is still very small
compared to the amount present in the remnant after 24 years, and significantly
higher dust masses at the earlier epochs are firmly ruled out by the
observations, indicating that the majority of the dust must have formed at very
late times. By 8515-9200 days after the explosion, 0.6--0.8\,M$_\odot$ of dust
is present, and dust grains with radii greater than 2\,$\mu$m are required to
obtain a fit to the observed SED. This suggests that the dust mass increase at
late times was caused by accretion onto and coagulation of the dust grains
formed at earlier epochs.
  These findings provide further confirmation that core collapse supernovae can
create large quantities of dust, and indicate that the reason for small dust
masses being estimated in many cases is that the vast majority of the dust
forms long after most supernovae have been detectable at mid-infrared
wavelengths.
