[Xperia Arc Q&A, Help & Troubleshooting] Nightglade:
I am slightly more coherent now. I'm not sure if this can help you, but this is the content of my:

status_bar_expanded_header.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout android:orientation="vertical" android:id="@id/header" android:background="@drawable/notification_header_bg" android:layout_width="fill_parent" android:layout_height="wrap_content"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui">
<View android:background="@drawable/status_bar_hr" android:layout_width="fill_parent" android:layout_height="2.0dip" />
<LinearLayout android:gravity="center_vertical" android:orientation="horizontal" android:layout_width="fill_parent" android:layout_height="@dimen/notification_panel_header_height" android:baselineAligned="false">
<com.android.systemui.statusbar.policy.Clock android:textAppearance="@style/TextAppearance.StatusBar.Expanded.Clock" android:id="@id/clock" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginLeft="8.0dip" android:singleLine="true" />
<com.android.systemui.statusbar.policy.DateView android:textAppearance="@style/TextAppearance.StatusBar.Expanded.Date" android:id="@id/date" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginLeft="8.0dip" android:layout_marginRight="8.0dip" android:maxLines="2" android:singleLine="false" />
<ImageView android:id="@id/settings_button" android:visibility="gone" android:layout_width="48.0dip" android:layout_height="48.0dip" android:src="@drawable/ic_notify_quicksettings" android:scaleType="center" android:contentDescription="@string/accessibility_settings_button" />
<Space android:layout_width="0.0dip" android:layout_height="0.0dip" android:layout_weight="1.0" />
<Button android:textAppearance="@style/TextAppearance.StatusBar.ClearButton" android:id="@id/clear_all_button" android:background="@drawable/btn_clear" android:paddingLeft="8.0dip" android:paddingRight="8.0dip" android:layout_width="100.0dip" android:layout_height="46.0dip" android:text="@string/status_bar_clear_all_button" android:layout_alignParentRight="true" android:contentDescription="@string/accessibility_clear_all" />
</LinearLayout>
</LinearLayout>

status_bar_expanded.xml

<?xml version="1.0" encoding="utf-8"?>
<com.android.systemui.statusbar.phone.ExpandedView android:orientation="vertical" android:focusable="true" android:descendantFocusability="afterDescendants"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui">
<com.lidroid.systemui.quickpanel.PowerWidget android:id="@id/quickpanel" android:layout_width="fill_parent" android:layout_height="wrap_content" />
<include android:layout_width="fill_parent" android:layout_height="@dimen/notification_panel_header_height" layout="@layout/status_bar_expanded_header" />
<FrameLayout android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_marginTop="2.0dip" android:layout_weight="1.0">
<TextView android:textAppearance="@*android:style/TextAppearance.Large" android:gravity="left" android:layout_gravity="top" android:id="@id/noNotificationsTitle" android:padding="8.0dip" android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="@string/status_bar_no_notifications_title" />
<ScrollView android:id="@id/scroll" android:fadingEdge="none" android:layout_width="fill_parent" android:layout_height="fill_parent" android:overScrollMode="ifContentScrolls">
<LinearLayout android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="wrap_content">
<com.android.systemui.statusbar.policy.NotificationRowLayout android:id="@id/latestItems" android:layout_width="fill_parent" android:layout_height="wrap_content" systemui:rowHeight="@dimen/notification_height" />
</LinearLayout>
</ScrollView>
</FrameLayout>
</com.android.systemui.statusbar.phone.ExpandedView>

and styles (/res/values/style.xml) relating to the expanded clock

<style name="TextAppearance.StatusBar.Expanded" parent="@*android:style/TextAppearance.Small.CalendarViewWeekDayView" />
<style name="TextAppearance.StatusBar.Expanded.Clock" parent="@style/TextAppearance.StatusBar.Expanded">
<item name="android:textSize">28.0dip</item>
<item name="android:textStyle">normal</item>
<item name="android:textColor">@*android:color/holo_blue_light</item>
</style>
<style name="TextAppearance.StatusBar.Expanded.Date" parent="@style/TextAppearance.StatusBar.Expanded">
<item name="android:textSize">11.0dip</item>
<item name="android:textStyle">normal</item>
<item name="android:textColor">#ffcccccc</item>
</style>

You said that you only edited status_bar_expanded. Did you try the status_bar_expanded_header? I've not personally played around with it, but the date is mentioned in that and not on status_bar_expanded.xml

You're also very likely to be missing the style associated with any attempt to add a date in the expanded header in a GB rom. I've not played around with GB nor do I have a phone with a GB rom on it, so I can't personally test this out. You can take a look at the code above me and see if that can help you figure anything out. If not, you're welcome to send me your SystemUI.apk and I'll take a look.

This might also be helpful for you

http://xdaforums.com/showthread.php?t=2384531