The DEP must be renewed biennially..., so if the original exemption was on February 3, 2004 does the renewal get filed in 2005 or 2006? I guess my confusion is what dates are considered "calendar year" for exemption purposes, January - December or March 16 - March 15th?

Jan-Dec. If the initial exemption was filed in an even year, the renewals will also be filed in even years.