The Microsoft Visual Studio 2013 Entity Framework database query does not support the subtraction of two dates to calculate TimeSpan.
For example the following statement will throw a runtime error:
site.VisitFrequencySec < ((TimeSpan)(dtNow - site.LastVisitDate)).TotalSeconds)
To solve the problem create a computed database column to calculate the difference between the dates and use the computed value for comparison.
Column definition:
[SecondsSinceLastVisit] AS (datediff(second,[LastVisitDate],getdate()))
Now you can use the computed column in the C# Entity Network comparison.
site.VisitFrequencySec < site.SecondsSinceLastVisit