Ok. Suppose you were born on October 1, 1972. On October1, 1995 you want
to know how old you are (it being your birthday and all). You subtract 1972
from 1995 to get 23. Whether you actually do it this way or not, this is
certainly one way to get the answer. You have been around now for 23 years. Happy birthday!!
I love the old Dickensian birthday greeting "many happy returns". My
grandparents used to fire that one at me.
Now, suppose you were born on January 1, 1AD (coincidentally, at the
beginning point of the current civil calendar). You live to a VERY
ripe old age and, on January 1, 2000AD, you want to know how old you are.
Exactly as above, you subtract 1 from 2000 to get 1999 years old. You won't
be 2000 years of age until January 1, 2001, a year later! Almost old enough
to legally enter a bar!
The point here is that calculating your age on your birthday is EXACTLY the
same as calculating the number of years that have passed on January 1 from
the beginning of the civil calendar.