Version 3.8 of the Android Newsgroup Downloader adds a new filter to the Article Chooser age filter called "Since Last Session". You can select it by using the dropdown button on the age filter. Here's a picture:

The "Since last session" filter displays that new posts that have arrived since the last time you loaded Android Newsgroup Downloader. Here is a quick rundown of how the feature works:

1. Whenever you view a newsgroup in the Article Chooser, Android saves the highest message number in that group to the "message pointer"
2. When you exit Android, the message pointer is saved to disk.
3. When you load Android again, Android retrieves the message pointer, and uses it for the "since last session" filter.

Many users have asked for this feature, as it's hard to remember where you "left off" last time you were browsing.