Need to Autofilter by date and include blank cells

So I have a spreadsheet that tracks computer equipment that is shipped out, and for each piece of shipped equipment (1 per row) an equivalent piece of equipment is returned to us. Each piece of return equipment is marked in the "date returned" column K.

I would like to set the autofilter options for column F to show only rows that have either:

A) A return date of the last 4 weeks
B) No return date at all

This will show me only rows with recently returned equipment, or equipment with pending returns.

I expected that I would be able to achieve this through customer filters by specifying:

"is after or equal to today-28" OR
"equal to <blank>"

So far I'm not having any luck, the autofilter box doesn't seem to respond to 'today-28'. Anyone know a way to achieve this?

probably the easiest way of doingthis is to add a helper column, and use a formula, something like...

=if(or(K1="",K1<=today()),"OK","")

and then filter on OK (you can use whatever you want instead of OK, as well as for the ""