While Julia is getting ready for the opera, the camera pans around the room and shows The British Press newspaper dated Tuesday, March 10, 1818. A couple of days after the opera, Julia writes a letter to Selina and dates it February 26, 1818.
In the dancing scene when a waltz is called none of the principal characters actually dance the waltz, which is a rather rigid form in 3/4 time that first became popular in Austria or Germany in the 1770s-1780s. In fact the diegetic music sort of fades out and becomes 4/4 background music to which the characters dance as a far-too-modern mundane 'slow-dance' number typical of the 1980s or later.