Read it again.
Exodus 12:18
In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening. The text identifies the fourteenth day at evening for the start of Unleavened Bread.
You said, "The 15th (beginning in the MORNING) is the first day of the feast of unleavened bread."
Here again is what the text actually states.
Exodus 12:18
In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread,
I read it. I agree that you don't eat leavened bread on Passover. That doesn't make Passover the first day of Unleavened bread.
Now you can read in Leviticus 23:
23 And the Lord spake unto Moses, saying,
2 Speak unto the children of Israel, and say unto them, Concerning the feasts of the Lord, which ye shall proclaim to be holy convocations, even these are my feasts.
3 Six days shall work be done: but the seventh day is the sabbath of rest, an holy convocation; ye shall do no work therein: it is the sabbath of the Lord in all your dwellings.
4 These are the feasts of the Lord, even holy convocations, which ye shall proclaim in their seasons.
5 In the fourteenth day of the first month at even is the Lord's passover.
6 And on the fifteenth day of the same month is the feast of unleavened bread unto the Lord: seven days ye must eat unleavened bread.