I can't seem to use a date field in a filter. Am I missing something?

No it is not possible yet.

Try to understand the way how Cook deals with filters and ou should easily create it be yourself.

TIP : if you already have a filter, search all instances of its name 'filter_xxxx' in your component. Because filters are not complicated, but are written few lines in maybe 4 or 5 files I don't remember. So just care to don't miss a part to be successful.

Then, if you really don't understand, I can take time with you.