Obviously the
select dt_bill_date from sc_bill where dt_bill_date>'2015-7-1' and sc_bill.st_bill_no=sc_bill_details.st_bill_no
query does not return any value.
I have searched for PostGRE datetime specification formats; have you tried to put two digits to the month and day numbers? I did not see anything about that, but all examples I can find use '2015-07-01' as date specification format rather than '2015-7-1'. I don't know the internals of PostGRE, nor did I ever develop anyhting based on this RDBMS, but it could be worth trying.
You could also change the comparison operator to
>=
and see if that returns some records.
You could also try with
select dt_bill_date
from sc_bill
inner join sc_bill_details on sc_bill.st_bill_no=sc_bill_details.st_bill_no
where dt_bill_date>'2015-07-01'
Hope this helps. Good luck.