I forgot to put my code in

Private Sub Detail_Format(Cancel As Integer, FormatCount As Integer)
On Error GoTo Err_Detail_Format

   ' Reset colors back to black on white
   Me![DTG].BackColor = RGB(255, 255, 255) 'white
   Me![DTG].ForeColor = RGB(0, 0, 0) 'black
 
   'Change the Colors
   If (IsNull(Me![DTG]) = True) Then GoTo Exit_Detail_Format
   Select Case Month(Me![DTG])
       Case 1
           Me![DTG].BackColor = RGB(0, 0, 255) 'blue
           Me![DTG].ForeColor = RGB(0, 0, 0) 'black
       Case 2
           Me![DTG].BackColor = RGB(180, 44, 30) 'maroon
           Me![DTG].ForeColor = RGB(255, 255, 255) 'white
       Case 3
           Me![DTG].BackColor = RGB(0, 255, 0) 'green
           Me![DTG].ForeColor = RGB(0, 0, 0) 'black
       Case 4
           Me![DTG].BackColor = RGB(255, 255, 19) 'yellow

etc, etc for all 12 months

Several things could be going on here.

Firstly, have you added your database folder as a trusted location under:
   Office Button | Access Options | Trust Center | Trust Center Settings

If you are working under Windows Vista, it may help to create a Run-As-Admin
shortcut. Details:
   http://allenbrowne.com/bug-17.html#RunAsAdmin

If you have done that, each version of Access uses a different binary for
the compiled code. So when you switch versions, you need to decompile. This
instructs Access to discard the binary code, and it will re-create it from
the text version of the code. Here's a standard rescue sequence that deals
with that and other issues. Try it in order:

1. Uncheck the boxes under:
   Tools | Options | General | Name AutoCorrect
In Access 2007, it's:
   Office Button | Access Options | Current Database | Name AutoCorrect
Explanation of why:
   http://allenbrowne.com/bug-03.html

2. Compact the database to get rid of this junk:
   Tools | Database Utilities | Compact/Repair
or in Access 2007:
   Office Button | Manage | Compact/Repair

3. Close Access. Make a backup copy of the file. Decompile the database by
entering something like this at the command prompt while Access is not
running. It is all one line, and include the quotes:
   "c:\Program Files\Microsoft office\office\msaccess.exe" /decompile
       "c:\MyPath\MyDatabase.mdb"

4. Open Access (holding down the Shift key if you have any startup code),
and compact again.

5. Open a code window.
Choose References from the Tools menu.
Uncheck any references you do not need.
For a list of the ones you typically need in your version of Access, see:
   http://allenbrowne.com/ser-38.html

6. Still in the code window, choose Compile from the Debug menu.
Fix any errors, and repeat until it compiles okay.

7. Still in the code window, choose Options on the Tools menu. On the
General tab, make sure Error Trapping is set to:
   Break on Unhandled Errors
and the Compile on Demand is unchecked.

At this point, you should have a database where the name-autocorrect errors
are gone, the indexes are repaired, inconsistencies between the text- and
compiled-versions of the code are fixed, reference ambiguities are resolved,
the code syntax is compilable, and the VBA options are set to show errors
and avoid this kind of corruption.

If it is still a problem, the next step would be to get Access to rebuild
the database for you. Follow the steps for the first symptom in this
article:
   Recovering from Corruption
at:
   http://allenbrowne.com/ser-47.html