Yeah, I tried doing it by date, but that doesn't really work since I uploaded a new image I found that I took back in the 90s and it didn't show in the feed because it was much older date wise (Exif info)... so I put the RSS feed back on ID. Filemtime might work.
But, after I get back to just doing my normal operation then it won't really matter as the ID will work fine because I typically look at the image right after I upload it anyway. Gallery, would process the image right after it was uploaded. I'm probably the only one that subscribes to it anyway so it's all a silly discussion most likely LOL. Anyway, still nice to know how it does things. Thanks for the info.