At the New Years Eve Party, "God Save the Queen" is sung, but at the time it should have been "God Save the King". (UK had a King from 1901 to 1952). The aeroplanes & motor cars would confirm time to be early 20th Century.
When Karen and Denys are dining, a wing type corkscrew, with two arms, is clearly shown on the table. Later in the same scene, Denys uses it to open a bottle of wine. This type corkscrew was invented in 1939.
Karen Blixen's champagne glass is empty when she slaps a rude man at the New Year's Eve party. Denys takes her arm and pulls her immediately onto the dance floor, but her glass has champagne in it again while they dance.