Yes, both could work in your case: "Last modified date" and "Added to album" - might worth experimenting which fits better.

Works for me. Make sure that in the field for the number of days, you have not entered any spaces around the number. That causes the "new" marking to fail.

(Laza - JSpinner!)

ETA: This applies only to the "new" marker. The search box option, on the other hand, does not appear to be working. It refuses to find images added today, no matter how many days I give it. I'll have to tinker to create a good test case for images that are 2, 3, 4, etc. days old.

Pretty much confirmed. Here's a little test album. All of the images were added on Mar. 23, about 12 days ago. The search doesn't find them, even when asking for images added in the last 30 days. I've tried various combinations (current folder, whole album, only one date, etc.), but can't find one that works.

https://jgromit.jalbum.net/bb/

ETA: Rebuilt with debug mode turned on.

Yes, that does work, but it shouldn't be necessary. You might want to be able to search for new images, but not have the thumbnails showing a "new" marker all the time. The two things should be independent of each other.

This is the main page of the album, not the "search results."