use Identity value in entity after insert in Database
I would suggest you change your database design. 

Option 1)

Have SQL Server set a column to the current date/time when you insert a row  See https://stackoverflow.com/q/6594589 for how to do it.  Then have that column mapped on your Tracking entity but mark the property as [DatabaseGenerated(DatabaseGeneratedOption.Computed)]  

Assuming you call the field AddDate

[DatabaseGenerated(DatabaseGeneratedOption.Computed)]
public datetime AddDate {get;set;}


Now you can create your get only TrackingCode property in code and use the [NotMapped] on it and define it as 

[NotMapped]
public int TrackingCode 
{
     get { return Convert.ToInt32(AddDate.ToString("yyyyMMdd") + tracking;
}


Option 2:

Create an after insert tigger and have SQL Server update the TrackingCode.  Again I would mark the property as DatabaseGenerated

In a nut shell it's better to offload this to the SQL Server