PM> Install-Package Ionad.Fody

[StaticReplacement(typeof(DateTime))]
public static class DateTimeSubstitute
{
    public static IDateTime Current { get; set; }

    public static DateTime Now { get { return Current.Now; } }
}

public void SomeMethod()
{
    var time = DateTime.Now;
    // ...
}

public void SomeMethod()
{
    var time = DateTimeSubstitute.Now;
    // ...
}