pgsql: Add time/date macros for code clarity: #define DAYS_PER_YEAR

Log Message:
-----------
Add time/date macros for code clarity:

#define DAYS_PER_YEAR 365.25
#define MONTHS_PER_YEAR 12
#define DAYS_PER_MONTH 30
#define HOURS_PER_DAY 24

Modified Files:
--------------
pgsql/src/backend/commands:
variable.c (r1.110 -> r1.111)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/variable.c.diff?r1=1.110&r2=1.111)
pgsql/src/backend/postmaster:
postmaster.c (r1.459 -> r1.460)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.459&r2=1.460)
syslogger.c (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/syslogger.c.diff?r1=1.16&r2=1.17)
pgsql/src/backend/tcop:
postgres.c (r1.454 -> r1.455)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.454&r2=1.455)
pgsql/src/backend/utils/adt:
date.c (r1.113 -> r1.114)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/date.c.diff?r1=1.113&r2=1.114)
datetime.c (r1.152 -> r1.153)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c.diff?r1=1.152&r2=1.153)
formatting.c (r1.91 -> r1.92)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/formatting.c.diff?r1=1.91&r2=1.92)
nabstime.c (r1.136 -> r1.137)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/nabstime.c.diff?r1=1.136&r2=1.137)
selfuncs.c (r1.185 -> r1.186)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.185&r2=1.186)
timestamp.c (r1.134 -> r1.135)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.134&r2=1.135)
pgsql/src/backend/utils/misc:
guc.c (r1.274 -> r1.275)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.274&r2=1.275)
pgsql/src/include/utils:
timestamp.h (r1.47 -> r1.48)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/timestamp.h.diff?r1=1.47&r2=1.48)
pgsql/src/interfaces/ecpg/pgtypeslib:
datetime.c (r1.22 -> r1.23)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/datetime.c.diff?r1=1.22&r2=1.23)
dt.h (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/dt.h.diff?r1=1.23&r2=1.24)
dt_common.c (r1.25 -> r1.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/dt_common.c.diff?r1=1.25&r2=1.26)
interval.c (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/interval.c.diff?r1=1.24&r2=1.25)
timestamp.c (r1.27 -> r1.28)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/timestamp.c.diff?r1=1.27&r2=1.28)