Great pattern created by Julie Lerman. This makes it easy to track data modification. The IsDirty property is mainly for Non-Disconnected applications.

EF-TrackDataHistory.cs

public override int SaveChanges(){
  foreach (var history in this.ChangeTracker.Entries()
    .Where(e => e.Entity is IModificationHystry && (e.State == EntityState.Added || e.State == EntityState.Modified))
    .Select(e => e.Entity as IModificationHistory)
    ){
    history.DateModified = DateTime.Now;
    if(history.DateCreated == DateTime.MinValue){
      history.DateCreated = DateTime.Now;
    }
  }
  int result = base.SaveChanges();
  foreach (var history in this.ChageTracker.Entries()
    .Where(e => e.Entry is IModificationHistory)
    .Select(e => e.Entity as IModificationHistory)){
      history.IsDirty = false;
  }
  return result;
}

public interface IModificationHistory 
{
  DateTime DateModified { get; set; }
  DateTime DateCreated { get; set; }
  bool IsDirty { get; set; }
}