The dating of many ancient Khmer and the significance of their alignment remain unclear. To correctly date ancient sites, the interdisciplinary fields of archaeoastronomy are required, as conventional archaeological methods are not sufficient. Many Khmer temples, as with other ancient civilizations, commonly used astronomical alignments in planning structures to address cosmic themes such as the cycle of life, death, and rebirth. In this study, the founding years of two ancient Khmer temples—Prasat Hin Phimai in Nakhon Ratchasima province and Prasat Hin Phanom Rung in Buriram province—were calculated and examined using archaeoastronomical techniques of dating. The azimuths of the two temples, being oriented off-east, do not correlate with the equinoxes or solstices, suggesting that the precise orientations were linked to particular star alignments that may have been related to their luni-solar calendar, festivals, and rituals. Applying astronomical techniques and modern equipment such as a theodolite, GPS, and Stellarium software, and taking precession into account, along with the azimuth and dates of the stars rising and setting, we find that the orientations of Prasat Hin Phimai and Prasat Hin Phanom Rung are related to the acronychal rising of the Pleiades and the cosmical setting of a bright star named 'Spica', respectively. The Pleiades rising had a connection with the traditional Lunar New Year's Day, today known in Thailand as Loy Kratong Day, while the Spica setting on the west-side doorway at dawn was used as a mark of the Sun entering the Mesha Rashi (Aries), known to be connected to the Saka Calendar's New Year's Day, or the current Songkran festival's Thaloeng Sok Day. By incorporating scientifically proven process as is the interdisciplinary nature of archaeoastronomy, we can gain greater accuracy and deeper knowledge of these ancient sites, and when they were likely constructed and in use.