How to change time in DateTime?

You can assign an initial value to a new DateTime value in many different ways:

• Extension Method

Extension method DateTime

    public static DateTime ChangeTime(this DateTime dateTime, int hours, int minutes, int seconds = default, int milliseconds = default)
    {
        return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day, hours, minutes, seconds, milliseconds, dateTime.Kind);
    }

then using ChangeTime:

    DateTime datetime = DateTime.Now; //Your DateTime
    datetime = datetime.ChangeTime(12, 20, 10);

• using the Add methods

  DateTime datetime = DateTime.Now; //Your DateTime
  datetime = datetime.Date.AddHours(12).AddMinutes(20).AddSeconds(10);
  

• using the Timespan

  DateTime datetime = DateTime.Now; //Your DateTime
  datetime = datetime.Date.Add(new TimeSpan(12, 20, 10));
  

• using initial value

  DateTime datetime = DateTime.Now;
  datetime = new DateTime(datetime.Year, datetime.Month, datetime.Day, 12, 20, 10);
  

In fact, you can't change the time once it's created. But you can create it easily with many constructors: https://docs.microsoft.com/en-us/dotnet/api/system.datetime.-ctor?view=netframework-4.7.2

For example, if you want to create a DateTime changing Seconds, you can just do this:

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

To set end of a day:

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