The following 'Sum' array formula works:
=SUM(IF($J$6:$J$500=DATEVALUE(TEXT(Q11,"mm/dd/yy")),IF($J$6:$J$500<=DATEVALUE(TEXT(R11,"mm/dd/yy")),$H$6:$H$500,0),0))

Unfortunately, the 'Counta' array formula version
=COUNTA(IF($J$8:$J$500=DATEVALUE(TEXT(Q11,"mm/dd/yy")),IF($J$8:$J$500<=DATEVALUE(TEXT(R11,"mm/dd/yy")),$C$8:$C$500,0),0))
doesn't seem to work.

What have I done wrong, or forgotten to do?