隨著新興國族國家的誕生，包括領土秩序在內的國際法規範，在內容有著根本性的轉變。在這規範基礎及內容與時俱進的過程中，如何確保國際法規範的穩定性與普遍性，俾使其能繼續維持為「舊的」與「新的」文明國家所願意共同遵守的行為準則，成為解釋國際法時必須面對的困題。戰後的去殖民化過程，更是國際法在可認知時空裡劇烈演進的一個現代適例。去殖民化的目的與結果，顛覆、更新了既有的國際法主體，同時否定了過去殖民帝國建立與維持國際社會秩序的主要手段，加上賦與殖民地依其意願變更領土權利的人民自決權，此時，除了必須以新的思維與想像對「舊的」規則重新詮釋外，也必須倚賴如時際法原則這類調和國際法秩序演進過程中價值衝突的解釋原則。 東南亞國家在戰前共有同一時空條件下的殖民歷史與遂生的文化言說，使得其彼此之間的領土爭議，成為在後殖民世界裡系統化時際法原則之實踐的天然場域，並提供了該原則被廣義化適用的各種例證。本文即試圖透過分析東南亞國家領土爭議中時際法原則的應用，包括對於「關鍵的時間點」、「殖民統治的有效性」、地圖文獻或非文獻式證據的解釋原則、以及國際強行法元素等適用條件的實踐，立證在現代國際法中，時際法原則的內容，其適用條件與限制。

Major context of international law, namely the territorial regulations, has envisaged a fundamental change since the emerge of nation-states. How to maintain the stability and universality of international law so to be followed at will by both the old and new civilized states then become a core issue for the international judiciaries while interpreting them. It is well noticed that decolonization per se is an instant example of such a drastic evolution of jus gentium. The purpose and outcomes of decolonization allow a gross renewal or renovation of the subject of international society, which together with the revocation of established rules of territorial acquisition made and maintained through the colonial period, forced the insertion of new thinking and imaginaries into the reading of the old rules. Nevertheless, utilities used to compromise the conflicting values among the old and new regulations, such as the doctrine of intertemporal law, is also proved to be indivisible. The Southeastern Asian states share the same colonial history and consequently the culture context, so the territorial disputes among them could help to serve as a natural forum to reveal the practical aspects of systemizing intertemporal law, including the broader interpretation and application of the doctrine. This article is therefore intending to deplore the major elements used to carve out the doctrine in the jurisprudence over the territorial disputes of The Southeastern Asian states, such as ”the critical date”, ”colonial effectivités”, maps and other documentary evidence, and jus cogens, so the substantial aspects, i.e., the applicability and limitation of the doctrine in modern international law may be further analyzed.