You can use strings to represent relative times:

&filter[fields][create_ts]=[NOW-5HOURS%20TO%20NOW]

or exact timestamps:

&filter[fields][create_ts]=[2011-06-01T05:00:00.00Z%20TO%202011-06-02T07:00:00.00Z]

or both:

&filter[fields][create_ts]=[2011-06-01T05:00:00.00Z%20TO%20NOW]