wiat.....
this is what you wrote:

>>>under reportexpression-->expression(plat),format ##:##:## ,common
>>>VAL(left(plan, 2))*3600 + VAL(SUBSTR(plan, 4, 2))*60 + VAL(SUBSTR(plan, 7, 2))

did you write the expression in the report field or you diddefine report variable as I adviced?

if it does not work in a whole, you'll need to divide the expressiom on parts and check every one separately.
try to write only VAL(SUBSTR(plan, 7, 2)) in the expression and look what result will this give you