Marc comes to pick up Anna and her nieces for dinner at Marc's mother's house and it is broad daylight. Next scene, on the drive to mother's house it is completely dark and they are enjoying the Christmas lights on display at the houses along the way (the drive time is not specified, so it is possible that it became dark on the drive). Furthermore, they specify the date as December 21, on or close to the Winter Solstice, and their pickup time as 5 PM. The New York City area is dark by that time on that date.
The movie takes place just before Christmas, but only time you see winter is in the New York City stock footage - bare deciduous trees, and a lot of snow. In the movie, all deciduous bushes and majority of trees are green (minority have yellowish leaves). Production sprayed some trees with artificial snow to hide it, and there was barely any snow anywhere except in the front of Marie's house. It was so warm, that in one scene when Mark comes in his Audi A4 car to to pick up Anna at her home, he actually drove it with the completely lowered driver's window.
Marc uses Anna's art in the advertising campaign without her permission. By doing this he violates copyright law (and sets himself up for a lawsuit). An advertising executive would know better.
The mom invites Mark to dinner and says Sunday night. After competing the call, he tells Anna they were invited to dinner tonight.
Marc is leaving his office and going home. He says to his secretary "Good night" except it was a broad daylight outside as can be seen clearly through the his office's window.