Update only Date Part

You can use the DateTime.ParseExact() method to parse the Start Time field, extract the time portion of it, and then combine it with the SessionStartTime field to create the desired result.

string sessionStartTimeStr = "2023/05/26";
string startTimeStr = "2023-05-19 17:00:00.000";
DateTime sessionStartTime = DateTime.ParseExact(sessionStartTimeStr, "yyyy/MM/dd", CultureInfo.InvariantCulture);
DateTime startTime = DateTime.ParseExact(startTimeStr, "yyyy-MM-dd HH:mm:ss.fff", CultureInfo.InvariantCulture);
DateTime result = sessionStartTime.Date + startTime.TimeOfDay;
string resultStr = result.ToString("yyyy/MM/dd HH:mm:ss.fff");

DateTime sessionStartTime = DateTime.Parse("2023/05/26 17:00:00.000");
DateTime newDate = DateTime.Parse("2023/05/26").Date;
TimeSpan timePart = sessionStartTime.TimeOfDay;
DateTime newDateTime = newDate + timePart;

// Now you can use the newDateTime value to update the SessionStartTime field in the database
 

In C#, you can use the `DateTime` and `TimeSpan` classes to update the date part of a `DateTime` value while keeping the time part the same. Here we first parse the original `SessionStartTime` value and extract the time part using the `TimeOfDay` property. We then parse the new date value and create a new `DateTime` value by combining the date part from the new date and the time part from the original `SessionStartTime` value. You can use the `newDateTime` value to update the `SessionStartTime` field in the database.

UPDATE your_table
SET SessionStartTime = CAST(CONVERT(date, SessionStartTime) AS datetime) + DATEPART(hour, [Start Time]) * 3600 + DATEPART(minute, [Start Time]) * 60 + DATEPART(second, [Start Time])

You can use the `DATEPART` function to extract the time part from the `Start Time` field and then combine it with the `SessionStartTime` field to create the final result. This query will convert the `SessionStartTime` field to a `datetime` value with the date part only, and then add the hour, minute, and second parts of the `Start Time` field to it. This will result in a `datetime` value with the same time as the `Start Time` field but with the date part of the `SessionStartTime` field.

Hello Ramco, 

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

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

• Add methods

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

• Timespan 

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

 BENCHAABAN Ali