現在時刻の取得をコード中に直に書かない ref: http://qiita.com/yuba/items/f52f90c4bd249d24b7d6

file2.cs

…
DateTime now = timeFetcher.Get();
…

file3.cs

class Time201401010900JST: ITimeFetcher {
    public DateTime Get() { return new DateTime(2014, 1, 1, 9, 0, 0); }
}
…
// アプリケーションを初期化。実行時刻は元日朝９時の設定。
AppCode app = new AppCode(new Time201401010900JST());
// アプリケーションをテスト
Test(app);
…

IEnvironmentFetcher.cs

public interface IEnvironmentFetcher {
    DateTime GetTime();
    …
}

ITimeFetcher.cs

public interface ITimeFetcher {
    DateTime Get();
}

TimeFetcher.cs

public class TimeFetcher: ITimeFetcher {
    public DateTime Get() { return DateTime.Now; }
}