Use ansi join syntax

Question

From what i have been reading is better to use the new syntax(dont know how new it is)

Answer 1

The ANSI join syntax was new to Oracle in Oracle 8i in 1998 - that's 15 years old.
It can produce more readable code, and is both more readable and less human-error prone for outer joins.
The ANSI format lets you outer join between multiple tables in a way the old oracle-specific ( + ) syntax did not and introduces FULL OUTER JOIN which you should use very rarely.
You should not use NATURAL JOIN in code - adding unrelated columns to the tables involved can make it give very different results.
There have not been significant bugs for ANSI joins in Oracle since Oracle 10.2 was introduced about 8 years ago.
As Paul said, the ON part should be the join criteria between the tables. The were clause should be the filtering of the joined tables.
So assuming start_date, end_date and in_property_id are variables
SELECT resv_num, unit_date
    FROM p_resv_unit ru
    INNER JOIN p_pm_unit_night pun
    ON pun.resv_unit_id = ru.resv_unit_id
    WHERE pun.property_id = in_property_id
    AND pun.unit_date BETWEEN start_date AND end_date
    AND pun.pm_unit_num = cvUnitNum;
If start_date and end_date were columns in p_resv_unit the query would be:
SELECT resv_num, unit_date
    FROM p_resv_unit ru
    INNER JOIN p_pm_unit_night pun
    ON pun.resv_unit_id = ru.resv_unit_id AND pun.unit_date BETWEEN ru.start_date AND ru.end_date
    WHERE pun.property_id = in_property_id
        AND pun.pm_unit_num = cvUnitNum;
Inner join queries work with criteria in the wrong place, but they're harder to read. Outer joins don't work unless you put the criteria in the right place.