Please find a small patch to fix the brain damage "century" and

"millennium" date part implementation in postgresql, both in the code
and the documentation, so that it conforms to the official definition.
If you do not agree with the official definition, please send your
complaint to "pope@vatican.org". I'm not responsible for them;-)

With the previous version, the centuries and millenniums had a wrong
number and started the wrong year. Moreover century number 0, which does
not exist in reality, lasted 200 years. Also, millennium number 0 lasted
2000 years.

If you want postgresql to have it's own definition of "century" and
"millennium" that does not conform to the one of the society, just give
them another name. I would suggest "pgCENTURY" and "pgMILLENNIUM";-)

IMO, if someone may use the options, it means that postgresql is used for
historical data, so it make sense to have an historical definition. Also,
I just want to divide the year by 100 or 1000, I can do that quite easily.

BACKWARD INCOMPATIBLE CHANGE

Fabien Coelho - coelho@cri.ensmp.fr

Author: bmomjian

doc/src/sgml/func.sgml

@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.196 2004/03/30 15:53:18 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.197 2004/04/10 18:02:59 momjian Exp $
 PostgreSQL documentation
 -->
 
@@ -4948,18 +4948,31 @@ EXTRACT (<replaceable>field</replaceable> FROM <replaceable>source</replaceable>
       <term><literal>century</literal></term>
       <listitem>
        <para>
-        The year field divided by 100
+        The historical definition of a century.
        </para>
 
 <screen>
-SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
+SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13');
 <lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
+SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>21</computeroutput>
 </screen>
 
        <para>
-        Note that the result for the century field is simply the year field
-        divided by 100, and not the conventional definition which puts most
-        years in the 1900's in the twentieth century.
+        An historical century is a period of 100 years.
+	The first century starts at 0001-01-01 00:00:00 AD, although
+	they did not know at the time. This definition applies to all
+	Gregorian calendar countries. There is no number 0 century, 
+	you go from -1 to 1.
+
+	If you disagree with this, please write your complaint to:
+	Pope, Cathedral Saint-Peter of Roma, Vatican.
+       </para>
+
+       <para>
+	Compatibility: if you want the previous postgres version of century,
+	just divide the year by 100. Note that with this definition, 
+	century number 0 lasts 200 years.
        </para>
       </listitem>
      </varlistentry>
@@ -5083,18 +5096,17 @@ SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
       <term><literal>millennium</literal></term>
       <listitem>
        <para>
-        The year field divided by 1000
+        The conventional historical millennium.
        </para>
 
 <screen>
 SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
-<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
+<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>
 </screen>
 
        <para>
-        Note that the result for the millennium field is simply the year field
-        divided by 1000, and not the conventional definition which puts
-        years in the 1900's in the second millennium.
+        Years in the 1900's are in the second millennium.
+	The third millennium starts January 1, 2001.
        </para>
       </listitem>
      </varlistentry>

src/backend/utils/adt/timestamp.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.103 2004/03/30 15:53:18 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.104 2004/04/10 18:02:59 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -3273,11 +3273,23 @@ timestamp_part(PG_FUNCTION_ARGS)
 				break;
 
 			case DTK_CENTURY:
-				result = (tm->tm_year / 100);
+				/* centuries AD, c>0: year in [ (c-1)*100+1 :     c*100   ]
+				 * centuries BC, c<0: year in [     c*100   : (c+1)*100-1 ]
+				 * there is no number 0 century.
+				 */
+				if (tm->tm_year > 0)
+					result = ((tm->tm_year+99) / 100);
+				else
+					/* caution: C division may yave negative remainder */
+					result = - ((99 - (tm->tm_year-1))/100);
 				break;
 
 			case DTK_MILLENNIUM:
-				result = (tm->tm_year / 1000);
+				/* see comments above. */
+				if (tm->tm_year > 0)
+					result = ((tm->tm_year+999) / 1000);
+				else
+					result = - ((999 - (tm->tm_year-1))/1000);
 				break;
 
 			case DTK_JULIAN:

src/test/regress/expected/date.out

@@ -819,3 +819,114 @@ SELECT date 'tomorrow' - date 'yesterday' AS "Two days";
         2
 (1 row)
 
+--
+-- test extract!
+--
+-- century
+--
+SELECT EXTRACT(CENTURY FROM DATE '0101-12-31 BC'); -- -2
+ date_part 
+-----------
+        -2
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '0100-12-31 BC'); -- -1
+ date_part 
+-----------
+        -1
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '0001-12-31 BC'); -- -1
+ date_part 
+-----------
+        -1
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '0001-01-01');    --  1
+ date_part 
+-----------
+         1
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '0001-01-01 AD'); --  1
+ date_part 
+-----------
+         1
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '1900-12-31');    -- 19
+ date_part 
+-----------
+        19
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '1901-01-01');    -- 20
+ date_part 
+-----------
+        20
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '2000-12-31');    -- 20
+ date_part 
+-----------
+        20
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM DATE '2001-01-01');    -- 21
+ date_part 
+-----------
+        21
+(1 row)
+
+SELECT EXTRACT(CENTURY FROM CURRENT_DATE)>=21 AS True;     -- true
+ true 
+------
+ t
+(1 row)
+
+--
+-- millennium
+--
+SELECT EXTRACT(MILLENNIUM FROM DATE '0001-12-31 BC'); -- -1
+ date_part 
+-----------
+        -1
+(1 row)
+
+SELECT EXTRACT(MILLENNIUM FROM DATE '0001-01-01 AD'); --  1
+ date_part 
+-----------
+         1
+(1 row)
+
+SELECT EXTRACT(MILLENNIUM FROM DATE '1000-12-31');    --  1
+ date_part 
+-----------
+         1
+(1 row)
+
+SELECT EXTRACT(MILLENNIUM FROM DATE '1001-01-01');    --  2
+ date_part 
+-----------
+         2
+(1 row)
+
+SELECT EXTRACT(MILLENNIUM FROM DATE '2000-12-31');    --  2
+ date_part 
+-----------
+         2
+(1 row)
+
+SELECT EXTRACT(MILLENNIUM FROM DATE '2001-01-01');    --  3
+ date_part 
+-----------
+         3
+(1 row)
+
+-- next test to be fixed on the turn of the next millennium;-)
+SELECT EXTRACT(MILLENNIUM FROM CURRENT_DATE);         --  3
+ date_part 
+-----------
+         3
+(1 row)
+

src/test/regress/sql/date.sql

@@ -208,3 +208,30 @@ SELECT date 'tomorrow' - date 'today' AS "One day";
 SELECT date 'today' - date 'yesterday' AS "One day";
 
 SELECT date 'tomorrow' - date 'yesterday' AS "Two days";
+
+--
+-- test extract!
+--
+-- century
+--
+SELECT EXTRACT(CENTURY FROM DATE '0101-12-31 BC'); -- -2
+SELECT EXTRACT(CENTURY FROM DATE '0100-12-31 BC'); -- -1
+SELECT EXTRACT(CENTURY FROM DATE '0001-12-31 BC'); -- -1
+SELECT EXTRACT(CENTURY FROM DATE '0001-01-01');    --  1
+SELECT EXTRACT(CENTURY FROM DATE '0001-01-01 AD'); --  1
+SELECT EXTRACT(CENTURY FROM DATE '1900-12-31');    -- 19
+SELECT EXTRACT(CENTURY FROM DATE '1901-01-01');    -- 20
+SELECT EXTRACT(CENTURY FROM DATE '2000-12-31');    -- 20
+SELECT EXTRACT(CENTURY FROM DATE '2001-01-01');    -- 21
+SELECT EXTRACT(CENTURY FROM CURRENT_DATE)>=21 AS True;     -- true
+--
+-- millennium
+--
+SELECT EXTRACT(MILLENNIUM FROM DATE '0001-12-31 BC'); -- -1
+SELECT EXTRACT(MILLENNIUM FROM DATE '0001-01-01 AD'); --  1
+SELECT EXTRACT(MILLENNIUM FROM DATE '1000-12-31');    --  1
+SELECT EXTRACT(MILLENNIUM FROM DATE '1001-01-01');    --  2
+SELECT EXTRACT(MILLENNIUM FROM DATE '2000-12-31');    --  2
+SELECT EXTRACT(MILLENNIUM FROM DATE '2001-01-01');    --  3
+-- next test to be fixed on the turn of the next millennium;-)
+SELECT EXTRACT(MILLENNIUM FROM CURRENT_DATE);         --  3