Combine TO_NUMBER and TO_DATE to get different parts of a Date : DATE « PL SQL Data Types « Oracle PL/SQL Tutorial

SQL>
SQL> set serveroutput on
SQL> set echo on
SQL> declare
  2    age   binary_integer;
  3
  4    birth_date    date;
  5    current_date  date;
  6
  7    function age_as_of (birth_date in date, as_of_date in date) return positive is
  8        as_of_year  natural;    --a year of 00 is valid.
  9        as_of_month positive;
 10        as_of_day   positive;
 11
 12        birth_year    natural;  --a year of 00 is valid.
 13        birth_month   positive;
 14        birth_day     positive;
 15
 16      age   positive;
 17    begin
 18        as_of_year := to_number(to_char(as_of_date,'yyyy'));
 19        as_of_month := to_number(to_char(as_of_date,'mm'));
 20        as_of_day := to_number(to_char(as_of_date,'dd'));
 21
 22        birth_year := to_number(to_char(birth_date,'yyyy'));
 23        birth_month := to_number(to_char(birth_date,'mm'));
 24        birth_day := to_number(to_char(birth_date,'dd'));
 25
 26        if as_of_month > birth_month then
 27           age := as_of_year - birth_year;
 28        elsif (as_of_month = birth_month) and (as_of_day >= birth_day) then
 29           age := as_of_year - birth_year;
 30        else
 31           age := as_of_year - birth_year - 1;
 32        end if;
 33
 34        return age;
 35    end;
 36  begin
 37
 38    current_date := to_date('11-13-1997','mm-dd-yyyy');
 39    birth_date   := to_date('11-15-1961','mm-dd-yyyy');
 40
 41
 42    age := age_as_of (birth_date, current_date);
 43    dbms_output.put_line(age);
 44
 45  end;
 46  /
35

PL/SQL procedure successfully completed.

SQL>
SQL>

21.6.DATE
	21.6.1.	DATE
	21.6.2.	Define DATE type variable
	21.6.3.	Date literals
	21.6.4.	Displaying Date and Time
	21.6.5.	Using the DATE Format Mask
	21.6.6.	Using Format Masks mixed with user string
	21.6.7.	Assignment value to DATE type variable during declaration
	21.6.8.	Use SYSDATE to initialize an DATE type variable
	21.6.9.	Define DATE type variable, assign value TRUNC(SYSDATE)
	21.6.10.	Use FOR LOOP to loop through dates
	21.6.11.	Compare DATE type variable in IF statement
	21.6.12.	Compare DATE type variable after truncating
	21.6.13.	Combine TO_NUMBER and TO_DATE to get different parts of a Date
	21.6.14.	Performing Calculations on a Converted Date
	21.6.15.	Converting DATE to a Spelled-Out Character Format
	21.6.16.	Calculate the months between two dates
	21.6.17.	Use the date value back and forth
	21.6.18.	Remember That Dates Are Numbers
	21.6.19.	Check the last date that a date type variable can hold
	21.6.20.	Use the new EXTRACT function to grab individual year and month components.
	21.6.21.	ROUND function being applied to datetime values
	21.6.22.	TO_CHAR(C,'YYYY-MM-DD HH:MI:SS.FF AM TZH:TZM TZR TZD')