pgsql: Update DAYS_PER_MONTH comment.

Log Message:
-----------
Update DAYS_PER_MONTH comment.

Add SECS_PER_YEAR and MINS_PER_HOUR macros.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        syslogger.c (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/syslogger.c.diff?r1=1.17&r2=1.18)
    pgsql/src/backend/utils/adt:
        date.c (r1.115 -> r1.116)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/date.c.diff?r1=1.115&r2=1.116)
        datetime.c (r1.154 -> r1.155)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c.diff?r1=1.154&r2=1.155)
        nabstime.c (r1.138 -> r1.139)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/nabstime.c.diff?r1=1.138&r2=1.139)
        timestamp.c (r1.137 -> r1.138)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.137&r2=1.138)
    pgsql/src/backend/utils/misc:
        guc.c (r1.275 -> r1.276)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.275&r2=1.276)
    pgsql/src/include/utils:
        timestamp.h (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/timestamp.h.diff?r1=1.50&r2=1.51)
    pgsql/src/interfaces/ecpg/pgtypeslib:
        dt.h (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/dt.h.diff?r1=1.26&r2=1.27)
        dt_common.c (r1.26 -> r1.27)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/dt_common.c.diff?r1=1.26&r2=1.27)
        interval.c (r1.26 -> r1.27)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/interval.c.diff?r1=1.26&r2=1.27)
        timestamp.c (r1.28 -> r1.29)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/pgtypeslib/timestamp.c.diff?r1=1.28&r2=1.29)