I have a LoginRecord table in sqlserver 2008 with the following column structure-
LoginId - int, identity
UserId - int
LoginDateTime- Allow nulls fals
Combined my two comments into an answer.
Try setting the "StoredGeneratedPattern" attribute of your datetime in the EDMX file to Computed. From the following thread: http://www.stackoverflow.com/a/4688135/2488939
To do this, go to the edmx file designer by clicking on your edmx file. Then locate your table and the property. Right-click the column in the table that you want to change and click on properties. The property window should then come up and you will see as one of the properties "StoredGeneratedPattern". Change that to computed.