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asp.NET and SQL Server datetime issues

I am having hard time to store date information into the datetime column of SQL Server.
I get the input from the user for three columns:
Creation Date
Preparation Date
Next Preparation Date
I use calendarextender and format the date as "yyyy/MM/dd". When all the fields have date, they are stored in the DB as for instance, 16-10-2016 (dd-MM-yyyy).
At this point I have two issues:
These columns are optional, when some of them are empty my code does not work (I assume because datetime cannot be null). To overcome this, I am using the following code snippet but still does not work.
DateTime? creationDate= null;
if (creationDateTextbox.Text != null && creationDateTextbox.Text != "")
{
creationDate= Convert.ToDateTime(creationDateTextbox.Text);
}
When I fetch the dates from DB, they are shown as 10/16/2016 (MM-dd-yyyy) which is different how I formatted it. I would like to show it in the format user enters them.

Dates do not have a format while stored in a database. It is actually usually just a very large long that counts the number of milliseconds from a set starting date.
If you want to store the format you need to stop storing it as dates and instead just treat the text as text in the database, however if you do this you won't get the advantage of sorting or filtering by a date range because it will just be seen as text.

Date time doesn't have any format You can format is as a string, suppose your DateTime type database field dt which contain date as 10/16/2016 (MM-dd-yyyy) then you can convert it
string s = dt.ToString("yyyy/MM/dd");

The answer to one of your questions is here: MSDN
You can use data annotations to format the dates that you get from your SQL DB. I'm assuming that you're using EF6; if not, you can change the field to a varchar in SSMS, and store the date as a String.
And the second, I'm unclear about, but if what you want is for your SQL DB column to be optional, you can use the Optional data annotation for that.

In Epplus Convert String Date as Date in Excel

I am dumping a string Date from DataTable to Epplus. I want to treat that date as Date in Excel. So that I can get Date Filter in Excel.
Things I tried,
Passed that date as Datetime. Now, Excel takes it as date and gets a date filter. But, it affects the pivot
Passed it as string with req. date format, The excel takes it as string and does not gets the date Filter in excel.
Hope i will get a solution from you guys. Any help will be appreciated.

You can use Numberformat property.
If you want set for whole column then you can apply it following way
ws.Column(1).Style.Numberformat.Format = "yyyy-MM-dd";
Or for particular cell
ws.Cells["A3"].Style.Numberformat.Format = "yyyy-MM-dd";

To Get Data that its type is Date in C#

In my SQL database, I have BeginDate column of type date (so I want to get only day,month,year).
I add data into database in C#.
I look at Table_Data and everything is fine (for example BeginDate = 05/04/2014).
But when I get this data from database in C# with
string a= dt.Rows[i]["BeginDate"].ToString()
result is: a= 05/04/2014 12:00:00 AM
Why?
I can remove hour in string a with any methods. But why? I want to get only 05/04/2014 from database.

This is a display problem, not a data problem. Use the Formatting features of DateTime.ToString() to display the date in the format that you want.
If the data is coming out of the database in string form, use DateTime.Parse on it first.
Reference
Standard Date and Time Formats
Custom Date and Time Formats

C# DateTime Convert from String

I am reading excel file in my C# .net web application and storing that value in Database. Ony of field in Excel is DateTime. I am storing it in string
string tran_time = Convert.ToString(odr[5]); //tran_time is "03-11-2012 16:08:43"
and then convert it in in DateTime and store it in Database (SQL Server 2008)
IFormatProvider culture = new CultureInfo("en-US", true);
DateTime dateVal = DateTime.ParseExact(tran_time, "dd-MM-yyyy HH:mm:ss", culture);
But the Value Being stored in Database is in format
2012-05-11 13:40:23.000 (yyyy-mm-dd)
and the value in Excel is 05-11-2012 13:40:23 (dd-mm-yyyy)
Date & Month is get replaced.
My Question is How can i store it in Database in Format (YYYY-MM-DD HH:MM:SS:FFF)

But the Value Being stored in Database is in format
No it isn't (assuming that you are using a DATETIME column type). This is just what the SQL tools show you.
A DateTime instance, either in a database or in C# does not have an associated format. It only gets formatted when displayed to the user.

Or use yyyy-MM-dd format. Works like a charm:
string myString = dateVal.ToString("yyyy-MM-dd");

Use dd-mm-yyyy instead of dd-MM-yyyy.

Propagate a DateTime column of a DataSet to database

I have a DataSet that has a timestamp (i.e. Datetime.Now ) recording the date and time when a row is added to the DataSet in memory. I will later save (or propagate) all these rows to the SQLCE database where the DataSet's timestamp will be propagate to a DateTime column in the database table.
It works fine and datetime comparison is also ok:
dataView1 = new DataView(
dt,
"DataTime >= '6/9/2011 5:00:20 PM'",
"Data_ID ASC",
DataViewRowState.CurrentRows);
The above code works ok, but I worry if the program runs on different computer (i.e. another language of Windows) the Datetime.Now format would have a different format, or if I compare data that is record from a different computer, the different format of the DateTime in the database will causes it to fail. Would this problem happen? Or is there a safer way of doing this?

String datetime = String.Format("{0:G}", ur_datetime_variable);
will give you string in M/d/yyyy h:mm:ss tt format and then convert the datetime string to DateTime
here's a link http://www.csharp-examples.net/string-format-datetime/ and there's everything bout datetime formating.

Should you not use CultureInvariant
DateTimeFormatInfo.InvariantInfo
when parsing dates