So it is a bit difficult to explain. Especially since someone can be born in Bosnia/Croatia and be a Serb or vice versa
A famous historical example of this would be Nikola Tesla, no?
His family lived in the area of modern day Croatia for hundreds of years. Nikola visited Serbia only once for 31 hours in total.
Yeah - its totally confusing. If Nikola was not Croatian after his family living in the area of Croatia for 500 years, how is Novak Djokovic Serbian for just being born in Serbia (while his parents are from Montenegro and Croatia)? :)
One can argue the usefulness of any of those distinctions, but there is going to be some genetic and some cultural part to the "nation" definition (around the Balkans, it's been the religion more than anything else to define a nation — like language, I would consider that a cultural component).
You seem to be going strictly for the geographical distinction (with time a contributing factor), but that's in opposition to how nations are defined everywhere (otherwise, it's all Croatians in Croatia today and there wouldn't exist any "Irish Americans", even with Americans being already a mash-up special nation). In Serbian people try to make the distinction by using "Srbi" and "Srbijanci" (citizens), basically Serb vs Serbians, though it is a linguistic stretch (just like we've got Bosniaks/Bošnjaci for a nation to oppose it to Bosnians/Bosanci which includes anyone from Bosnia and Herzegovina).