Why does the birth year not count?

Because it ties the number to a whole cohort. Day and month are more personal and describe your own contribution more precisely.

Can the mission change during a life?

The number stays. What changes is how much room you give it.

Then read it beside the consciousness and destiny numbers. On its own, every number reads narrower than it is.

The mission number comes from the day and month of your birth. It describes what you put your strength into when the choice is yours.

Example: 23/07 becomes 2+3+0+7 = 12, and 1+2 = 3. The mission number is 3.

The day describes your immediate manner, the month the frame it shows up in. Together they give the direction your energy takes by itself.

The year is deliberately left out: it ties you to an entire cohort and says little about your personal contribution.

The mission number names no profession. It describes a way of working that fits inside many of them.

Two people with the same mission number can do entirely different work and still be doing the same thing.

That is common and not a fault. Usually it is enough to move a small part of the everyday toward the mission number for something to give.

What each number says about contribution, strength and shadow side.

Why does the birth year not count?: Because it ties the number to a whole cohort. Day and month are more personal and describe your own contribution more precisely.
Can the mission change during a life?: The number stays. What changes is how much room you give it.
What if my mission number feels alien?: Then read it beside the consciousness and destiny numbers. On its own, every number reads narrower than it is.

The calculator shows consciousness, destiny and mission together, with their meanings. Free.