Since
IMP XV is
rare in either known combination, with TR P
VII as in
RIC or with TR P
VIII as shown on
Forvm by Tacrolimus and David A., I'd say it deserves to be called not just a
rare dating combination, but an outright
rare date. Apparently
Domitian only held that title for a short time, because news of
his next acclamation,
IMP XVI, arrived almost immediately.