The good news (I just heard this a very short while ago) is that the "show hands based off of the last X hands" will be available in the next beta. Before you go feel like this time was wasted, though, I should be able to use what you've done here to create additional filters for the Filters area for things like today, yesterday, last week, this week, etc. Those won't make it into the Beta due out tomorrow, but they should make it into the Beta after that.
The "last x hands" filter will disable the cache, though, so may cause performance issues for people, so I'm guessing the problem with the HUD and your stats is cache related also. What happens in cache creation, as far as I'm aware, is that PT3 goes through each stat and calculates it as it stands right then. Then when the stat is needed in the HUD, it calls the cached stat and adds in any new values since the cache was created.
So, let's say you're looking at the stat VP$IP (as it's simple). It's defined as cnt_vpip / cnt_hands. When the cache runs through VP$IP, it sees that it needs two columns: cnt_vpip and cnt_hands. The cache thus has a cnt_vpip and cnt_hands column which is calculated for each player at the time of the caching. When you're playing later and a player is on the table it gets cnt_vpip and cnt_hands from the cache for that player, and adds in any new hands afterwards.
So what's happening to your stats, I think, is that when the cache is run, it's saying "what was 15 min ago". Then when you play later, it's adding in the new stuff (they're not 15 min old yet), and then the stats likely aren't being dropped later (so if you played for 30 min with the 15 min ago filter, you'd probably have 30 in of data, I think). That's my best guess anyway.
Either way, this information is still useful and thanks to you there will definitely be filter for this month filters available in PT3 most likely in Beta 24. (I still need to build and test the filters, but you've given me all the tools to put it together fairly quickly I think.) And very shortly you'll have the filter you wanted in the first place
