Error in c# subtracting 30 days from current date ? winforms

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C#

SQL

WinForms

MS-Access

I need to subtract 30 days from current date then i filter query but i didn't get any results/ output why whats my error on this code

DateTime curdate = DateTime.Now;
curdate = curdate.AddDays(-30); // if i give -4 instead of -30 the query will bind data
DateTime curdate1 = DateTime.Now;

validateDept.InitializeConnection();
OleDbConnection connection = new OleDbConnection(validateDept.connetionString);
OleDbDataAdapter adapter = new OleDbDataAdapter("SELECT InvoiceId, InvoiceNumber, InvoiceDate, (Select CustomerId from Customer Where Customer.CustomerId=NewInvoice_1.CustomerName) AS CustomerId, (Select CustomerName from Customer where Customer.CustomerId = NewInvoice_1.CustomerName) AS CustomerName, DueDate, Tax, GrandTotal, CompanyId FROM NewInvoice_1 WHERE InvoiceDate >= '" + curdate + "' AND InvoiceDate <= '" + curdate1 + "' ", connection);
DataSet sourceDataSet = new DataSet();
adapter.Fill(sourceDataSet);
gridControl1.DataSource = sourceDataSet.Tables[0];

empty table only shows if i run this code. If i change -30 to -4 then it fetch one row from Access DB. From 1st april to current date 8th april if we give -3,-4,-5,-6, -7 this code but works small error is that "<" & ">" this only working "=" sign not working in this code ?

Thanks a lot.