public interface IFoo
    {
        protected static string GetFormattedTime() => DateTime.Now.ToString("HH:mm");

        public void Foo();
        public void Bar();
    }

    public class Baz : IFoo
    {
        public void Foo() { ... }
        public void Bar() => Console.Writeline(IFoo.GetFormattedTime())
    }

    public interface IFoo
    {
        protected static string GetFormattedTime() => DateTime.Now.ToString("HH:mm");

        public void Foo();
        public void Bar();
    }

    public class Baz : IFoo
    {
        public void Foo() { ... }
        public void Bar() => Console.Writeline(IFoo.GetFormattedTime())
    }