Aad is sometimes taken to mean the beginning. It actually means more like the primal, something that existed before the beginning, beyond and predating anything. This concept is very hard to translate into English. Pooran means complete, needs nothing else to complete it. In the beginning, God was complete truth. Madh pooran, in the middle, God is completely whole, completely true, all pervading. Ant Pooran, in the end God will be all pervading. Parmaysureh. Why? Because he is parmaysureh. He is the highest of high, highest of the world. This is a lot like Guru Nanak Dev ji's words at the beginning of Japji Sahib. Aad Sach, Jugaad Sach, Hai Bhee Sach. Naanak Hosee Bhee Sach.