The Collaborative International Dictionary of English v.0.48:

Datary \Da"ta*ry\, n. [LL. datarius. See Dataria.]
   1. (R. C. Ch.) An officer in the pope's court, having charge
      of the Dataria.
      [1913 Webster]

   2. The office or employment of a datary.
      [1913 Webster]

The Devil's Dictionary (1881-1906):

DATARY, n.  A high ecclesiastic official of the Roman Catholic Church,
whose important function is to brand the Pope's bulls with the words
_Datum Romae_.  He enjoys a princely revenue and the friendship of
God.