using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var start = "2021-05-05 10:00";

        var stop = "2021-05-05 12:00";

        var diff = TimeBetween(start, stop);

        Console.WriteLine(diff);

    }

    public static string TimeBetween(string startTime, string stopTime)

    {

        DateTime start = Convert.ToDateTime(startTime);

        DateTime end = Convert.ToDateTime(stopTime);

        TimeSpan ts = end - start;

​

        var hours = Convert.ToInt32(ts.TotalHours);

        var minutes = ts.Minutes;

        var sign = (hours < 0 || minutes < 0) ? "-" : "";

​

        hours = Math.Abs(hours);

        minutes = Math.Abs(minutes);

​

        var result = sign + hours.ToString() + ":" + minutes.ToString();

        return result;

    }

}

​