using System;

public class Program

{

    public static void Main()

    {

        var startTime = new DateTime(2019, 01, 01, 4, 0, 0);

        var endTime = new DateTime(2019, 01, 01, 16, 0 , 0);

        Console.WriteLine(GetTime(0.5, startTime, endTime));

    }

    private static string GetTime(double percentage, DateTime startTime, DateTime endTime)

    {

        var diff = (endTime - startTime).TotalMinutes;

        double minutes = diff * percentage;

        var result = startTime.AddMinutes(minutes);

        return result.ToShortTimeString();

    }

}