Someone else might be able to use a better way, but this works. Make sure that the Analysis ToolPak is added (Tools, Add Ins..., put a check mark next to Analysis Toolpak). Put this formula in another cell say C13 (you can hide the column from your sheet if necessary):
=IF(EOMONTH($A$1,0)=EOMONTH(B13,0),1,0)
Which puts a 1 if the months are the same, else 0.
Then you can use this formula in your conditional format in B13:
=C13=1
For some reason you can't put the EOMONTH formula directly in the conditional format, I'm guessing probably because it is an add-in formula.
When I go and click to add the Analysis ToolPak, it requires to load it from the CD. But for some reason it doesn't find a file it needs on the CD, data1.msi. I am back to square one.
This worked for me.
Highlight the entire worksheet, then put this as a formula in conditional formatting
=AND(MONTH(A1)=MONTH($A$1),YEAR(A1)=YEAR($A$1))
Derek
data1.msi is on all MS Office installation cd's, it just may be hard to find. You will need to search the CD to find where the file is located (Start -> Find -> Files or Folders. Change the drive letter to your CD Rom drive letter and search in all directories for the filename). You will probably want to take the time do this because the Analysis ToolPak is very useful.