How can I label content in views as "new"?
I have some suggestions, ranked below sort of from simple to complex.


You can simply add a static text field to the view, write "New" in the text field, and then filter the view to contain only content from the period you want to have the "New" label on them.
If you want the same list to also display more content, create an attachment display in the same view, override the text field and the date filter, and set the display to attach after the first one.
Use the Theme information in the view to directly override the views output. Find the content id in the template, load the node, and check the creation date.
Render the content through a view mode instead of on a field by field basis. In the the node-template, you can now check the creation date of the node and output the label. This solution would possibly benefit from an added 'per-view-mode' theme suggestion.