DateTime in entity Framework && DateTime in sql format returning different results

Guys i have a big problem in entity framework and DateTime. Here the problem:

First with entity:

When the user submit the form them those values are being create for a condicional query depending on which parameter the user type. Example:

var query =  from user in UserTable

select user;

        if(Request.Form["Category"])
        {
        query = query.where(add=>add.Category == Request.Form["Category"])
        }

//and here the between 
         if(Request.Form["start_date"] && Request.Form["end_date"])
        {
        query = query.where(add=>add.Date < Request.Form["start_date"] && add.Date > Request.Form["end_date"] )//equivalent to between
        }
    until...
    return query.ToList();

Good, but the problem remains that when the user wants to search by date, when converting the request form to datetime then the format are different from the input what was recived:

User'input
2003-12-02


Convert.ToDateTime
result : 02/11/2003(not iqual to the user input)

So, there is not coincidence(Well, thats what i guest because I read that entity solved this by itself). And i cannot set Date propery to String Because i cannot use between with a string.

Here the model simplified:

public string Category {get;set;}
public DateTime Date{get;set;} 

But Thats not all

Part 2

I tried to query in 02/11/2003 format to see if the query works and it did correctly, but when i tried with 2003-12-02, here the results

 02/12/2003 format
16330 rows

2003-12-02 format
136 rows

Notice that they are the same but return different results. Moreover, when i check the results, i noticed that when querying in 02/12/2003 format the results include date lesser or highter:

Example:

Select date FROM Table WHERE date BETWEEN 02/12/2003 AND  03/12/2003

results include: 05/12/2003,30/12/2003

Why this significant diffents bettwen those two queries?