Enhance documentation of date/time functions.  Add extract, current_date, current_time, current_timestamp.  Add more examples.  Remove age(), because it doesn't work like it is documented, and no one could explain it to me.
Enhance documentation of date/time functions.  Add extract, current_date,
current_time, current_timestamp.  Add more examples.  Remove age(), because
it doesn't work like it is documented, and no one could explain it to me.

Quikling/gpdb,0x0FFF/gpdb,royc1/gpdb,Chibin/gpdb,janebeckman/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,Chibin/gpdb,greenplum-db/gpdb,0x0FFF/gpdb,greenplum-db/gpdb,ahachete/gpdb,Quikling/gpdb,Quikling/gpdb,ahachete/gpdb,adam8157/gpdb,edespino/gpdb,Chibin/gpdb,rvs/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,chrishajas/gpdb,CraigHarris/gpdb,cjcjameson/gpdb,atris/gpdb,edespino/gpdb,janebeckman/gpdb,yazun/postgres-xl,arcivanov/postgres-xl,rvs/gpdb,kmjungersen/PostgresXL,lisakowen/gpdb,jmcatamney/gpdb,kmjungersen/PostgresXL,zeroae/postgres-xl,Chibin/gpdb,randomtask1155/gpdb,cjcjameson/gpdb,greenplum-db/gpdb,rvs/gpdb,Postgres-XL/Postgres-XL,tpostgres-projects/tPostgres,xinzweb/gpdb,snaga/postgres-xl,chrishajas/gpdb,0x0FFF/gpdb,Quikling/gpdb,ovr/postgres-xl,oberstet/postgres-xl,edespino/gpdb,zeroae/postgres-xl,oberstet/postgres-xl,lintzc/gpdb,chrishajas/gpdb,lisakowen/gpdb,foyzur/gpdb,postmind-net/postgres-xl,zeroae/postgres-xl,50wu/gpdb,greenplum-db/gpdb,zaksoup/gpdb,50wu/gpdb,randomtask1155/gpdb,royc1/gpdb,kaknikhil/gpdb,zeroae/postgres-xl,foyzur/gpdb,cjcjameson/gpdb,cjcjameson/gpdb,Chibin/gpdb,xuegang/gpdb,xinzweb/gpdb,janebeckman/gpdb,tangp3/gpdb,Quikling/gpdb,rubikloud/gpdb,CraigHarris/gpdb,Postgres-XL/Postgres-XL,lpetrov-pivotal/gpdb,rubikloud/gpdb,Chibin/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,tangp3/gpdb,50wu/gpdb,randomtask1155/gpdb,randomtask1155/gpdb,CraigHarris/gpdb,tpostgres-projects/tPostgres,lintzc/gpdb,kaknikhil/gpdb,lintzc/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,kaknikhil/gpdb,adam8157/gpdb,rubikloud/gpdb,lpetrov-pivotal/gpdb,50wu/gpdb,rvs/gpdb,postmind-net/postgres-xl,zaksoup/gpdb,rvs/gpdb,rubikloud/gpdb,royc1/gpdb,pavanvd/postgres-xl,foyzur/gpdb,foyzur/gpdb,lpetrov-pivotal/gpdb,lpetrov-pivotal/gpdb,tangp3/gpdb,jmcatamney/gpdb,postmind-net/postgres-xl,CraigHarris/gpdb,Quikling/gpdb,greenplum-db/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,postmind-net/postgres-xl,adam8157/gpdb,xinzweb/gpdb,CraigHarris/gpdb,tpostgres-projects/tPostgres,yuanzhao/gpdb,chrishajas/gpdb,yazun/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,xuegang/gpdb,tangp3/gpdb,jmcatamney/gpdb,chrishajas/gpdb,edespino/gpdb,greenplum-db/gpdb,ahachete/gpdb,royc1/gpdb,pavanvd/postgres-xl,randomtask1155/gpdb,yuanzhao/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,randomtask1155/gpdb,lisakowen/gpdb,ovr/postgres-xl,arcivanov/postgres-xl,Chibin/gpdb,ashwinstar/gpdb,royc1/gpdb,chrishajas/gpdb,atris/gpdb,arcivanov/postgres-xl,kmjungersen/PostgresXL,rvs/gpdb,chrishajas/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,rubikloud/gpdb,yuanzhao/gpdb,zaksoup/gpdb,jmcatamney/gpdb,xuegang/gpdb,atris/gpdb,oberstet/postgres-xl,atris/gpdb,jmcatamney/gpdb,lpetrov-pivotal/gpdb,pavanvd/postgres-xl,cjcjameson/gpdb,rvs/gpdb,yuanzhao/gpdb,50wu/gpdb,rvs/gpdb,ashwinstar/gpdb,janebeckman/gpdb,Chibin/gpdb,snaga/postgres-xl,arcivanov/postgres-xl,adam8157/gpdb,lpetrov-pivotal/gpdb,lisakowen/gpdb,ovr/postgres-xl,edespino/gpdb,Quikling/gpdb,pavanvd/postgres-xl,janebeckman/gpdb,jmcatamney/gpdb,tpostgres-projects/tPostgres,Chibin/gpdb,kaknikhil/gpdb,lpetrov-pivotal/gpdb,ovr/postgres-xl,xuegang/gpdb,CraigHarris/gpdb,xinzweb/gpdb,chrishajas/gpdb,lintzc/gpdb,ahachete/gpdb,kmjungersen/PostgresXL,royc1/gpdb,snaga/postgres-xl,tangp3/gpdb,lisakowen/gpdb,xinzweb/gpdb,cjcjameson/gpdb,xinzweb/gpdb,lisakowen/gpdb,royc1/gpdb,edespino/gpdb,royc1/gpdb,zaksoup/gpdb,Postgres-XL/Postgres-XL,cjcjameson/gpdb,yazun/postgres-xl,xuegang/gpdb,oberstet/postgres-xl,yazun/postgres-xl,ahachete/gpdb,lintzc/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,atris/gpdb,techdragon/Postgres-XL,techdragon/Postgres-XL,0x0FFF/gpdb,edespino/gpdb,lintzc/gpdb,snaga/postgres-xl,lintzc/gpdb,oberstet/postgres-xl,tangp3/gpdb,janebeckman/gpdb,0x0FFF/gpdb,zeroae/postgres-xl,foyzur/gpdb,CraigHarris/gpdb,0x0FFF/gpdb,atris/gpdb,rubikloud/gpdb,lintzc/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,zaksoup/gpdb,kaknikhil/gpdb,janebeckman/gpdb,yuanzhao/gpdb,0x0FFF/gpdb,rubikloud/gpdb,xinzweb/gpdb,foyzur/gpdb,xuegang/gpdb,randomtask1155/gpdb,greenplum-db/gpdb,arcivanov/postgres-xl,edespino/gpdb,edespino/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,postmind-net/postgres-xl,50wu/gpdb,rvs/gpdb,atris/gpdb,ashwinstar/gpdb,rvs/gpdb,CraigHarris/gpdb,zaksoup/gpdb,lintzc/gpdb,CraigHarris/gpdb,arcivanov/postgres-xl,yazun/postgres-xl,0x0FFF/gpdb,janebeckman/gpdb,xuegang/gpdb,xuegang/gpdb,greenplum-db/gpdb,edespino/gpdb,ahachete/gpdb,zaksoup/gpdb,yuanzhao/gpdb,50wu/gpdb,Quikling/gpdb,kmjungersen/PostgresXL,janebeckman/gpdb,ashwinstar/gpdb,50wu/gpdb,tangp3/gpdb,xuegang/gpdb,foyzur/gpdb,rubikloud/gpdb,tpostgres-projects/tPostgres,foyzur/gpdb,atris/gpdb,Quikling/gpdb,zaksoup/gpdb,ahachete/gpdb,tangp3/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,Chibin/gpdb,janebeckman/gpdb,Quikling/gpdb,ovr/postgres-xl,snaga/postgres-xl,randomtask1155/gpdb,ahachete/gpdb,lpetrov-pivotal/gpdb,yuanzhao/gpdb,techdragon/Postgres-XL