Reading Date column from database

I have table:

CREATE TABLE zakresy (
    z_ID integer NOT NULL  PRIMARY KEY AUTOINCREMENT,
    poczatek date NOT NULL,
    koniec date NOT NULL
)

My problem is that I can't fetch data from that table. I'm using code:

SQLiteDataReader Rzakresy = new SQLiteCommand("SELECT poczatek, koniec FROM zakresy", CDB).ExecuteReader();
            while (Rzakresy.Read())
            {
                DateTime DTpoczatek = (DateTime)Rzakresy[1]; // throw exception
                DateTime DTkoniec = (DateTime)Rzakresy[2];
            // ... 
            }

Date in table are stored like "2015-03-14"

I'm struggling for half hour but nothing works :(