这是一种贫民区的方法，但它很有效:)

DateTime dt = DateTime.Now; //get a DateTime variable for the example
string newSecondsValue = "00";
dt = Convert.ToDateTime(dt.ToString("MM/dd/yyyy hh:mm:" + newSecondsValue));

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);

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

设定一天的结束:

date = new DateTime(date.Year, date.Month, date.Day, 23, 59, 59);

试试这个

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

我更喜欢这个:

DateTime s = //get some datetime;
s = new DateTime(s.Year, s.Month,s.Day,s.Hour,s.Minute,0);