Syntax error using preparedstatements with postgresql

I have a Java application connecting to a PostgreSQL database.

Using straightforward code like:

        ps = con.prepareStatement(sql);
        if (dataTypes != null && dataTypes.size() > 0) {
            for (int i = 0; i < dataTypes.size(); ++i) {
                if (dataTypes.get(i) == DataTypes.NUMERIC)
                    ps.setLong((i + 1), Long.parseLong(values.get(i)));
                else
                    ps.setString((i + 1), values.get(i));
            }
        }
        rs = ps.executeQuery();

and with a query like

select ? from dummy

it just works fine. In the postgres logs what shows is:

2015-06-13 12:48:30 EEST [28294-3] xx LOG:  execute <unnamed>: select $1 from dummy
2015-06-13 12:48:30 EEST [28294-4] xx DETAIL:  parameters: $1 = '0'

But with a query like:

select to_char(now()+interval ? day, 'YYYYMMDD') from dummy

I get a syntax error:

Cause [org.postgresql.util.PSQLException: ERROR: syntax error at or near "$1"

and in the postgres logs:

2015-06-13 13:17:39 EEST [29311-3] xxx ERROR:  syntax error at or near "$1" at character 31
2015-06-13 13:17:39 EEST [29311-4] xxx STATEMENT:  select to_char(now()+interval $1 day, 'YYYYMMDD') from dummy

but in pgadmin something like

select to_char(now()+interval '0' day, 'YYYYMMDD') from dummy

works just fine without any error.

I just can't figure out what the problem is. Why does one of the queries work but the other one doesn't?

My PostgreSQL version is 9.4