where p_project_code = 10
and p_date = to_date('10-Apr-2017')
and case 
when p_company = 0 
then company -- All Companies when parameter 0 will pass
else p_company --or it will display only OneCompany based on parameter pass.
end;

where p_project_code = 10
and p_date = to_date('10-Apr-2017')
and company = case 
when p_company = 0 
then company -- All Companies when parameter 0 will pass
else p_company --or it will display only One Company based on parameter pass in p_company.
end;

SQL> select to_date('10-Apr-2017') from dual;
select to_date('10-Apr-2017') from dual
               *
ERROR at line 1:
ORA-01858: a non-numeric character was found where a numeric was expected