Hi,
First, if you look at the data in the database for the order_modified column, you'll see that the format is a timestamp.
So you need to enter a timestamp in your filter:
en.wikipedia.org/wiki/Unix_time
Also, when you use a trigger on a after/before modification/creation event, only the elements modified/created will be filtered.
So having a filter on the product_modified >= to today midnight will do the same as no filter at all, since all the products you'll create/modify will already have then product_modified greater than today midnight.