Datetime does not show up

Why does the timestamp not show in the application. I have tried the onCreate query in different ways and also tried to to have addTime as a value in addPrime. Nothing seems to work. My intention is for the app to show previous primes and the time that they were found. The intention for the app is for the user to be able to close/kill the app and resume counting from last found prime number when restarting the app, if you have any hints how also this would be possible I would be grateful.

This is the PrimeDBManager class

    package com.example.dator.prime3;

import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.Cursor;
import android.content.Context;
import android.content.ContentValues;


public class PrimeDBManager extends SQLiteOpenHelper {

    private static final int DATABASE_VERSION = 1;
    private static final String DATABASE_NAME = "prime.db";
    public static final String TABLE_PRIME = "prime";
    public static final String COLUMN_ID = "_id";
    public static final String COLUMN_PRIMENO = "primeno";
    public static final String COLUMN_DATETIME = "datetime";


    public PrimeDBManager(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, DATABASE_NAME, factory, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String query = "CREATE TABLE " + TABLE_PRIME + "(" + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + COLUMN_PRIMENO + " TEXT " + COLUMN_DATETIME + " DATETIME DEFAULT CURRENT_TIMESTAMP " + ");";
        db.execSQL(query);

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_PRIME);
        onCreate(db);

    }

    //Add a new prime to the database
    public void addPrime(Prime prime){
        ContentValues values = new ContentValues();
        values.put(COLUMN_PRIMENO, prime.get_primeno());

        SQLiteDatabase db = getWritableDatabase();
        db.insert(TABLE_PRIME, null, values);
    }

    public void addTime(Prime prime) {
        ContentValues values = new ContentValues();
        values.put(COLUMN_DATETIME, prime.get_datetime());
        SQLiteDatabase db = getWritableDatabase();
        db.insert(TABLE_PRIME, null, values);
       }

    public String databaseToString(){
        String dbString = "";
        SQLiteDatabase db = getWritableDatabase();
        String query = "SELECT * FROM " + TABLE_PRIME + " WHERE 1";


        Cursor c = db.rawQuery(query, null);

        c.moveToFirst();
        while(!c.isAfterLast()) {
            if (c.getString(c.getColumnIndex("primeno"))!=null){
                dbString += c.getString(c.getColumnIndex("primeno"));
                dbString += "\n";
        }
            c.moveToNext();
        }
        db.close();
        return dbString;
    }

}

Prime class

    package com.example.dator.prime3;

public class Prime {

    private int _id;
    private String _primeno;
    private String _datetime;



    public Prime(){

    }

    public Prime(String _primeno) {
        this._primeno = _primeno;
    }

    public void set_id(int _id) {
        this._id = _id;
    }

    public void set_primeno(String _primeno) {
        this._primeno = _primeno;
    }

    public int get_id() {
        return _id;
    }

    public String get_primeno() {
        return _primeno;

    }


    public void set_datetime(String _datetime) {
        this._datetime = _datetime;

    }

    public String get_datetime() {
        return _datetime;
    }


}

And lastly the MainActivity class

package com.example.dator.prime3;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;




public class MainActivity extends ActionBarActivity {



    Button primeButton;
    int max = 500;
    TextView primeText;
    int j = 2;
    TextView previousPrime;
    PrimeDBManager dbManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        primeButton = (Button) findViewById(R.id.primeButton);
        primeText = (TextView) findViewById(R.id.primeText);

        previousPrime = (TextView) findViewById(R.id.previousPrime);
        dbManager = new PrimeDBManager(this, null, null, 1);
        printDatabase();


        primeButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                for (int i = j; i <= max; i++) {

                    if (isPrimeNumber(i)) {
                        primeText.setText(i+"");
                        j = i+1;

                        break;

                    }
                }
                Prime prime = new Prime(primeText.getText().toString());
                dbManager.addPrime(prime);
                dbManager.addTime(prime);
                printDatabase();


            }
        });

    }

    public void printDatabase () {
        String dbString = dbManager.databaseToString();
        previousPrime.setText(dbString);


    }

    public boolean isPrimeNumber(int number) {

        for (int i = 2; i <= number / 2; i++) {
            if (number % i == 0) {
                return false;
            }
        }
        return true;
    }


}