Have you tried setting the date as a formatted date?
http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html#function%5Fstr-to-date[
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MySqlDataAdapter da = new MySqlDataAdapter("Select * from Transactions WHERE date BETWEEN STR_TO_DATE('"+datePicker1.Text +"','%d,%m,%Y') AND STR_TO_DATE('"+datePicker2.Text +"','%d,%m,%Y') ", cs);
This is what you would probably do in normal SQL.
I also noticed that both between dates are from DatePicker1.Text... was that intentional?