LEBP,
How do you fix it? Don't use a date field. Instead use an extra number field for both your calculated value and to replicate your start date. For example use the CSng() function to convert your 1 month look-ahead date to a numeric value in the Number1
field. And, use another extra number field to capture and convert the Start field to a numeric value. Now filter on those two custom number fields.
Strange? You bet. Project 2010 has multiple little glitches that were not present in previous versions. For example, your approach works fine in Project 2007. This little glitch may or may not be addressed by a hotfix in the cumulative updates. I know
it's not addressed by SP-1 because I have that installed. If you want to take a look at the issues handled by cumulative updates, go to, http://technet.microsoft.com/en-us/office/ee748587.
Wish I had a better answer for you but at least you now have a workaround.
John