Hey Everyone - Be careful using "Is before" and "in past before" date field operators. Marketo Support has just informed me that it is expected behavior for the date field operators "is before" and "in past before" to qualify records where the date field is empty!
In other words - suppose the person email@example.com was never synced to SFDC. This Smart List does not come up empty. You have to add an additional filter for SFDC Created Date is not empty.
The same thing applies to this example:
Obviously this isn't my real use case. I'm just using the above as a simplified way to describe the situation. I discovered this behavior when a couple of segments I created having to do with end dates for certain products contained unreasonably large numbers of people. I will be creating an idea around this and will add a link to it to this thread when I do.