After publishing page 42 (Sales Order) as a web service in NAV, I added a web reference to the newly created web service in my Visual Studio project. In the C# code, I create a new instance of the service, and tell it to use the default credentials:
SalesOrders_Service salesOrdersService = new SalesOrders_Service();
salesOrdersService.UseDefaultCredentials = true;
Then I instantiate a filter, and set the field and criteria:
SalesOrders_Filter filter = new SalesOrders_Filter();
filter.Field = SalesOrders_Fields.Document_Date;
filter.Criteria = "01-31-14| "; // specific date (MM-dd-yy) or empty
The filter instance is then added to a new array of SalesOrders_Filters before passing the latter to ReadMultiple:
SalesOrders[] salesOrders = salesOrdersService.ReadMultiple(new SalesOrders_Filter[] { filter }, null, 0);
On my machine, this returns two orders whose Document Date is 31 January 2014, and one order with a blank Document Date.