On Thanksgiving, Kelly says she moved home six months ago. Days before Christmas which is approximately one month later, Kelly says that she moved home six months ago, not seven as would be the case if the Thanksgiving statement is true.
The movie takes place just before Christmas, and it was shot in the winter. However, there is a small difference between mid December and late February / early March - you can hear loud singing by singing birds in the few outdoor scenes, and they don't sing between early autumn and late February / early March.
Dax says he is from Beatrice, Nebraska, but at the dinner table the family mispronounces the town by saying: BEE ah tress. It is actually pronounced Bee AT tress.
The fireplace in Harding's house is faux. It always has the three same logs, and fire burns in the same loop.
Time zone differences show up twice. First in the opening scene, both Kelly (Illinois) and Dax (NYC) are just waking up. For Kelly it is 9:00 AM, for Dax it is 2:00 PM. Then when Dax wakes up from his nap in Illinois and is talking to his girlfriend in NYC, it is 6:00 for him and 8:00 for her. Illinois is in Central Time Zone, and NYC is in Eastern Time Zone. They are only one hour different.