pass date type data to PQexecparams

I need to pass date type data from one table to another using the libpq
library wich meens that I'm using the C programming language...
PQexecparams look's like:

res = PQexecParams(conn,
"INSERT INTO orders
(userid,productid,qty,nomprice,discount,totprice,delivered,orderdate) VALUES
($1::int4,$2::int4,$3::int4,$4::int4,$5::int4,$6::int4,$7::int4,$8::QUESTION)",
8,
NULL,
paramValues,
paramLengths,
paramFormats,
1)

You can see that last 8th argument is of type QUESTION, well this is my
question wich datatype must be declared to pass date into ?