How to use DuckDuckGo to do a basic ‘what’s new in the last month’ search of all posts at WordPress.com blogs…
site:wordpress.com/2018/06 “sir gawain”
All posts have the same date-based URL structure, so that’s why it works. Be aware that DuckDuckGo will freak out if you do a search like this more than five times, and assume that you’re some kind of Killer Web-Bot and temporarily block access. Also works with Google, though you may get an annoying “are you a robot?” captcha. Increasingly, doing any remotely sophisticated and intensive search is now deemed dangerous. /sigh/.
You can do the same for Blogger-based blogs:
site:blogspot.com/2018/06/ “sir gawain”
Though with far less success, in terms of relevance and spam. No wonder Google dislikes blogs and thinks them ‘spammy’, if it judges them by its own blogging service.
As for timing, you have to assume that indexing may be delayed by up to two weeks for sporadic blogs, so a regular middle-of-the-month search across the previous month is probably the best option.
WordPress does have its own search-engine, which claims to run across all its blogs, but in comparison with Google it doesn’t seem very comprehensive.