Elinor's birth year, revealed on screen in the Dashwood family bible as 1781, along with her stated age of 19 years firmly establishes the setting of the story in 1800. Therefore Willoughby's discussion with Marianne about Lord Byron's poetry is anachronistic because Byron was born in 1788 and is only twelve years of age at the time his poetry is being discussed by the characters.
When Margaret and Marianne are taking their walk to get away from Cl Brandon, Margaret is talking to Marianne and says "Don't you like him Elinor?"
About 42 minutes in, Marianne is playing the piano, and the camera pans across some manuscript music which is upside down.