This is done using a function called fs/modified-since: https://blog.michielborkent.nl/speeding-up-builds-fs-modified-since.html

Thanks! I reckon I'll progressively play with it I use Makefiles extensively. I also am fostering Makefiles for users here https://github.com/clojure-emacs/enrich-classpath/tree/b57f1659f3fed36e4f05f1a8fc5706eb0329efd8#any-lein-or-toolsdeps-project but I reckon bb would have a greater acceptance factor (plus easier Windows compat)

Is there something I should know about this feature? Any outstanding issues?

it's not a feature, just a function you use like any other function

or do you mean bb tasks itself? I haven't come across any bugs in a long while

the latter