When I do that, I only get January 2014 data. So I tried something and put both years in one grouping like this.
if {PRJC.PREndDate} in cdate(2013,01,05) to cdate(2013,01,26) then 'January' else
if {PRJC.PREndDate} in cdate(2013,02,02) to cdate(2013,02,23) then 'February' else
if...