Buddhist Day of Celebration in cambodia

I think everybody know and understand about the date of the Khmer New Year but many people do not know why we take the 13th, 14th, and 15th of April as the beginning of the New Year. This is not a very hard question to answer if we answer this three questions:
What is the time concept of the Khmer people? When did we start to use it? How do we count time? Before I talk about these things, I would like to mention that: Khmer New Year is not a religious or Buddhist celebration (Bon Put SaS-NA), even though, most Khmer people go to the pagoda at that time and give food to the monk, do Poun Phnom Ksach ceremony, etc. All of these do not mean that Khmer New Year is a Buddhist celebration.
There is a big difference between a traditional ceremony and a religious ceremony or a Buddhist ceremony. Some traditional ceremonies are religious ceremonies also but not all are religious, as in the case of the Khmer New Year.