This article will describe a way on how to change/set a primary display on CentOS/RHEL 7 with dual monitors and GNOME. The easiest and perhaps recommended way is to use GUI and navigate to Applications->System Tools->Settings->Displays.
show current primary display redhat/centos

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The current primary screen is highlighted by a black top bar. To change you primary display simply drag-n-drop the black top bar from one screen to another:
change primary display redhat/centos
. Another alternative is to you command line tool xrandr. First get all your connected screens:
$ xrandr | grep -w connected
DVI-D-1 connected 1680x1050+0+0 (normal left inverted right x axis y axis) 474mm x 296mm
HDMI-1 connected primary 1920x1080+1680+0 (normal left inverted right x axis y axis) 598mm x 336mm

When not using NTP you may need to set your system time manually. You have two options to set time and date on your RHEL7 linux. First option is to use date command to do this job or engage dedicated systemd timedatectl command. By default and without any arguments timedatectl will display a current time, local, universal and RTC times:
[root@rhel7 ~]# localectl
   System Locale: LANG=en_AU.iso88591
       VC Keymap: us
      X11 Layout: us
[root@rhel7 ~]# timedatectl
      Local time: Thu 2014-09-04 18:30:11 WST
  Universal time: Thu 2014-09-04 10:30:11 UTC
        RTC time: Thu 2014-09-04 10:30:10
        Timezone: Australia/Perth (WST, +0800)
     NTP enabled: n/a
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a

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