试试这个

var NewDate = Convert.ToDateTime(DateTime.Now.ToString("dd/MMM/yyyy")+" "+"10:15 PM")/*Add your time here*/;

DateTime出了什么问题。AddSeconds方法，您可以添加或减去秒?

s = s.Date.AddHours(x).AddMinutes(y).AddSeconds(z);

这样你就可以保留你的日期，同时根据你的喜好插入新的时、分、秒部分。

 Using an extencion to DateTime:  

        public enum eTimeFragment
        {
            hours,
            minutes,
            seconds,
            milliseconds
        }
        public static DateTime ClearTimeFrom(this DateTime dateToClear, eTimeFragment etf)
        {
            DateTime dtRet = dateToClear;
            switch (etf)
            {
                case eTimeFragment.hours:
                    dtRet = dateToClear.Date;
                    break;
                case eTimeFragment.minutes:
                    dtRet = dateToClear.AddMinutes(dateToClear.Minute * -1);
                    dtRet = dtRet.ClearTimeFrom(eTimeFragment.seconds);
                    break;
                case eTimeFragment.seconds:
                    dtRet = dateToClear.AddSeconds(dateToClear.Second * -1);
                    dtRet = dtRet.ClearTimeFrom(eTimeFragment.milliseconds);
                    break;
                case eTimeFragment.milliseconds:
                    dtRet = dateToClear.AddMilliseconds(dateToClear.Millisecond * -1);
                    break;
            }
            return dtRet;

        }

像这样使用:

游戏机。WriteLine (DateTime。Now ClearTimeFrom (eTimeFragment时间)。

这个必须返回: 2016-06-06 00:00:00.000

int year = 2012;
int month = 12;
int day = 24;
int hour = 0;
int min = 0;
int second = 23;
DateTime dt = new DateTime(year, month, day, hour, min, second);

事实上，时间一旦创建就不能更改。 但是你可以用很多构造函数轻松创建它: https://learn.microsoft.com/en-us/dotnet/api/system.datetime.-ctor?view=netframework-4.7.2

例如，如果你想创建一个DateTime变化的秒，你可以这样做:

DateTime now = DateTime.Now;
DateTime secondschanged = new DateTime(now.Year, now.Month, now.Day, now.Hour, now.Minute, yourseconds);