Searching by date range

[YoungFrankendata]

I'm trying to get imapfilter to select messages with a date range and then move them into a specific folder on Yahoo Mail. Yahoo's imap server doesn't return results for UID SEARCH ALL BEFORE or UID SEARCH ALL SENTBEFORE so the method I was going to try was this:
results = yahoo.Inbox:arrived_since('01-Jan-2018') - yahoo.Inbox:arrived_since('01-Jan-2019')

My thoughts were that this would return all messages that were in the first search without the messages in the second search. I have verified that the searches themselves return results by inspecting the imap debug log. However, when the command to move is executed it does nothing and there is no error message. It appears that the result set is empty, or at least that's how it seems to me.

Do I misunderstand the 'not' operator as it's explained in the sample config file and the man page? Any help would be appreciated.

[YoungFrankendata]

UPDATE: After poking around more today, I now understand that the root cause is Yahoo IMAP's 10,000 message limit. Therefore, any complex result sets must cover the same 10,000 message range in order to work as expected. This same limitation is what was affecting UID SEARCH ALL BEFORE and UID SEARCH ALL SENTBEFORE since I was using dates from four to five years ago. Both work when the dates cover messages within the 10,000 message limit.