using System;
using System.Globalization;

namespace DateMergeTime
{
    class Program
    {
        static void Main(string[] args)
        {
            var _date = DateTime.Now;

            string[] _horas = { "17:12:1", "161023", "17:20", "17:2", "1720", "172", "17", "xablau" };

            foreach (var hora in _horas)
            {
                Console.WriteLine(_date.SetTimeFromString(hora));
            }
        }
    }

    public static class DateTimeHelpers
    {
        public static DateTime SetTimeFromString(this DateTime date, string time)
        {
            return date.WithoutTime() + time.ToTimeSpan();
        }

        public static DateTime WithoutTime(this DateTime datetime)
        {
            return new DateTime(datetime.Year, datetime.Month, datetime.Day);
        }

        public static TimeSpan ToTimeSpan(this string time)
        {
            time = time.Replace(":", "");

            time = time.Length <= 5
                   ? time.PadRight(6, '0')
                   : time;

            if (TimeSpan.TryParseExact(time, "hhmmss", CultureInfo.InvariantCulture, out TimeSpan parsedTime))
                return parsedTime;

            return new TimeSpan();
        }
    }
}