I’ve had similar issues and reported it. The ultimate conclusion was that n days ago is a range not a specific date so comparing it the way you are doesn’t work. What I would do is Crest a Salesforce field that returns true if today minus created date is less than 15 days. Then you can create a condition in your model based on whether that field is true or false.

Skuidward is correct and the post he is referring to is this:
https://community.skuid.com/t/next-n-weeks-not-working

We have it catalogued with our developers and are working on putting out an enhancement.

Hmmm…this workaround of creating the boolean formula field ((TODAY() - CreatedDate) <= 15) and using it as the condition doesn’t seem to be working…when the condition is met…i.e. created in last 15 days, the record pulls up. However, when the condition is not met, the’ Create default row if model has none’ is not creating a new row.

I think I may have found something…the condition works when “Load Model data on Page Load” is checked…but not when the model is loaded through an action on the page (in my case an action on the tab). 

In fact, it’s not just the boolean workaround…it seems to work even with the Last_N_Days:15 condition.

So it appears the issue is with the tab action that Queries the Model rather than the Last_N_Days:15 condition…see links to screencasts below

Buggy: http://recordit.co/RRtzA5m82b

Working: http://recordit.co/wSAImiKXX8