How can I filter to only null/empty dates in jqGrid?

I'm building an ASP.NET MVC 3 application and using the jqGrid to display tabular data. In one of my grid displays I show recipe data and there are a couple of date columns where I don't always have a date for the data.

Using the date picker I can fil开发者_Go百科ter to things that have a particular date or happen before/after a particular date. But how can I filter to things that are missing a date? The workaround I'm using right now is to sort the date column and that'll get me easily enough to the missing dates but I'd like to be able to easily answer the question "How many recipes are missing the last prepared date?"

Any ideas for this?