Tribunicia potestate

I found this passage in Wikipedia [and have deleted it for now]
http://en.wikipedia.org/wiki/Tribune :

As a result, there was never actually an "office" of emperor; emperors'
reigns were dated by their assumption of ''tribunicia potestas'', and the
actual constitutional position of the emperor was that of ''pontifex
maximus'' (P.M.) ''tribunicia potestate'' (trib. pot.).

I know it is not true that Tiberius' reign was dated by his assumption of
tribunicia potestas [yikes! that would put the start of his reign a decade
earlier] and so forth, but was it true in the late Empire? And "actual
constitutional position" was that of ''pontifex maximus'' (P.M.)
''tribunicia potestate'' (trib. pot.)"?

Ling Ouyang
http://janusquirinus.org/