As it turns out there were two conflicting policies being applied to the system. My account was not affected by one of the policies so everything seemed to work fine for me, however when any other user account tried (because those accounts were being affected by the other policy) the screensaver policy was overwritten with the policy that caused what I posted as the "Actual Result".

Unfortunately that other policy is a very restrictive policy so we couldn't access gpresult or rsop to figure this out sooner. Using Group Policy Results helped us figure it out.

Yes, it is possible to change the lock screen date format.

You will need Microsoft Locale Builder.

Resources

Read about locales here:

• Custom Locales (Windows) - MSDN:
  https://msdn.microsoft.com/en-us/library/windows/desktop/dd317785(v=vs.85).aspx

Download Microsoft Locale Builder here:

• Download Locale Builder 2.0 from Official Microsoft Download Center:
  http://go.microsoft.com/fwlink/p/?linkid=96654

Instructions

1. Locale Builder > Next > Dates > Long Date Patterns > Lock screen date pattern: dddd, d MMMM
2. Then, go to Build > Build Zip Batch File > Save.
3. Extract the ZIP file.
   • You can see that the generated LDML file contains the element msLocale:lockScreenFormat.
4. Run the CMD file within an administrator command prompt.

Apply locale

If you do not see any changes after installing the custom locale, you may need to apply the custom locale or reset your region settings.

Windows 10 and newer:

• Settings > Time and language > Date & time > Additional date, time, & regional settings > Under "Region", click "Change date, time, or number formats" > Additional settings... > Reset > Yes > OK > OK.

Windows 8 and earlier:

• Control Panel > Under "Clock, Language, and Region", click "Change date, time, or number formats" > Additional settings... > Reset > Yes > OK > OK.

Windows:

• Start > Run > intl.cpl > Additional settings... > Reset > Yes > OK > OK.

Notes and limitations

• If you are missing the day of the week, month, or day notation, the lock screen will discard the custom date format pattern, and the lock screen will format manually with the full month name, comma, space, and the partial month day (element msLocale:monthDay; Dates > Partial Date Patterns > Month day pattern) pattern.

  The following symbols work:

  | Symbol | Meaning         | Presentation |  Example |
  |--------|-----------------|--------------|----------|
  |    M   | Month in year   | Text, number | July, 07 |
  |    d   | Day in month    | Number       |       10 |
  |    t   | AM/PM marker    | Text         |       PM |
  |    G   | Era designator  | Text         |       AD |
  |    '   | Escape for text | Delimiter    |          |
  

  The following symbols do not work:

  | Symbol | Meaning              | Presentation | Example |
  |--------|----------------------|--------------|---------|
  |    y   | Year                 | Number       |    2009 |
  |    h   | Hour in AM/PM (1-12) | Number       |      12 |
  |    H   | Hour in day (0-23)   | Number       |       0 |
  |    m   | Minute in hour       | Number       |      30 |
  |    s   | Second in minute     | Number       |      55 |
  

• Microsoft Locale Builder only supports pattern values less than or equal to 79 characters:

  The property 'LockScreenDatePattern' must be less than or equal to 79 characters in length.

• You can have multiple repeating patterns.

• Long patterns will be wrapped onto the next line on the lock screen.