I am most certainly looking forward to the new variable-filtering feature in the upcoming release!
Regarding data driven categories or the soon-to-be variable-filtering, an expression like {File.DateTime|filter:YYYY:MM} groups the date - down to the month. Just for that one variable.
How can I create an expression that will group that way, but with respect to two, three, or however many date variables I so choose?
Your question is not clear. Do you want to group based on multiple dates at the same time?
You can do so, with regular data-driven categories based on tags. If you want to group by multiple dates on the same level, use multiple variables for your data-driven category, and combine them as you need them.
Okay.
Consider the |format:YYYYMM argument that can be included in {File.DateTime|format:YYYYMM}.
But, what would be the correct expression to have all files - with date 201410 - be considered for each and every one of the date variables - ie., date modified, date created, etc.?
I assume your dates are in a mess and you don't have valid dates in all files in the standard two XMP date tags?
That's not really good.
Filling the two standard XMP date tags consistently is a very important factor for proper asset management in IMatch - because IMatch uses the two dates for so many important things. From the timeline to events to improving face recognition quality.
If you want to group your files by arbitrary dates, I assume you want to look at multiple dates and choose a non-empty one? Or the best one?
You can use the hasvalue:Text (https://www.photools.com/help/imatch/#var_basics.htm?dl=h-223) formatting function to pick the first of your various dates with a value, but there is no way to code "best" date.