Shloshim is thirty days of mourning. We don’t mourn as long as we don’t know that he died. Only after burial, which is Stimas Hagolel, do the rules of mourning apply. Therefore, anyone that gets buried on a later date (for many reasons) the Shloshim will end later as well. The only case where the Shloshim ends earlier is when it meets a Regel (holiday). There is another scenario where a Shloshim might not apply at all, when a relative (for example an elderly person) is notified at a later date (of the burial).