using System;

public class Program

{

    public static void Main()

    {

        DayOfWeek backupDayOfWeek = DayOfWeek.Thursday;

        TimeSpan backupTime = new TimeSpan(8,0,0);

        DateTime nextDate = DateTime.Today.AddDays((7 + (backupDayOfWeek - DateTime.Today.DayOfWeek)) % 7);

        DateTime nextDateTime = nextDate.AddHours(backupTime.Hours).AddMinutes(backupTime.Minutes).AddSeconds(backupTime.Seconds);

        if (nextDateTime < DateTime.Now)

            nextDateTime = nextDateTime.AddDays(7);

        Console.WriteLine($"next datetime {nextDateTime}");

        TimeSpan dateDiff = nextDateTime - DateTime.Now;

        Console.WriteLine($"milliseconds {dateDiff.TotalMilliseconds}");

    }

}