Correct DateTime format in SQL Server CE?

If you're worried about getting the format right at all, something has already gone seriously wrong. There are two things you need to do to correctly work with datetime values in any database, not just sqlce:

1. Make sure you're using a datetime type for the column (not a text type like varchar)
2. Make sure you're using a datetime parameter in a parameterized query, and not string concatenation.

If you do that, there is no formatting involved on your part. At all. Example:

 void SetDate(int recordID, DateTime timeStamp)
 {
    string SQL = "UPDATE [sometable] SET someDateTimeColumn= @NewTime WHERE ID= @ID";

    using (var cn = new SqlCeConnection("connection string here"))
    using (var cmd = new SqlCeCommand(SQL, cn))
    {
        cmd.Parameters.Add("@NewTime", SqlDbType.DateTime).Value = timeStamp;
        cmd.Parameters.Add("@ID", SqlDbType.Integer).Value = recordID;

        cn.Open();
        cmd.ExecuteNonQuery();
    }
} 

Never ever ever ever EVER use string manipulation to substitute values into sql queries. It's a huge no-no.

Try the following format:

DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss")