Is the constellation calculated by solar calendar or lunar calendar?

You said the change is because you look for the corresponding Gregorian birthday according to the lunar birthday every year, of course, every year is different.

Your Gregorian birthday is actually the Gregorian calendar date of the year you were born, and it is unchanged. You shouldn't look for your own Gregorian birthday every year. Your birthday is the Gregorian calendar date of the year when you were born.

If you don't remember,

You can check the date conversion table of the lunar calendar online.

Enter lunar birthday

Even if the Gregorian date comes out.