How to create a text file and save in windows mobile..?

First of all I don't understand what is File.Create for?
Second, You're creating FileStream with mode FileMode.Truncate. This mode according to MSDN:
"Specifies that the operating system should open an existing file. When the file is opened, it should be truncated so that its size is zero bytes. This requires FileIOPermissionAccess.Write permission. Attempts to read from a file opened with FileMode.Truncate cause an ArgumentException exception."

I'm assuming you're getting FileNotFoundException, because file "date.txt" does not exist, which is required by FileMode.Truncate.

Instead of using FileStream, you can do this in easier way:

using (var swFile = new StreamWriter("date.txt", false, Encoding.ASCII))
{
    swFile.Write("new text");
}

In this case StreamWriter has 3 parameters:
1. Path to file
2. Append (bool)
2.1 If is set to True, and file exists it will append data to end of file
2.2 If is set to False, and file exists it will open file, truncate it's contents and set its position to 0
2.3 If file not exists in both cases it will create new file and set its position to 0.
3. Encoding.

By using using clause you don't have to close stream, beacuse StreamWriter implements IDisposable interface and at the end of using block it will be closed automatically.

Here you have some useful informations:
1. http://msdn.microsoft.com/library/system.io.filemode%28v=vs.110%29.aspx[^]
2. http://msdn.microsoft.com/pl-pl/library/system.io.streamwriter%28v=vs.110%29.aspx[^]

Hope it helps you a little bit.