Asslam o alikum.
In Islam the new day begins after maghrib , right.
When we do sehri and close our fast at fagr , we do the intention(niyat) of fast. From fagr to maghrib(when we open the fast) it is the same day. But the intention(niyat) of fast is:
I intend to keep the fast for tomorrow in the month of Ramadan.
What is the reason behind this? Thank you.