Formula to Compute One's Exact Age

Date: 05/31/2000 at 11:39:32
From: Orlando Haddock
Subject: Formula/Equations

Is there an equation to find a person's exact age on a given date 
(taking into account leap years and all)? I guess you need to set up 
some sort of equation like N = d + m + y where d = the day of the 
month, m = the month of the year, and y = the year. I tried some of 
the answers you have given to similar questions, but have not been 
able to get the right result. Please help me or point me in the right 
direction.

Thank you so much for your time and assistance.
Orlando

Date: 05/31/2000 at 13:55:51
From: Doctor Rob
Subject: Re: Formula/Equations

Thanks for writing to Ask Dr. Math, Orlando.

Yes, one's exact age in days can be computed based on the date of 
birth and the current date. We can assume that both dates are given in 
the Gregorian calendar, that in common usage today.

Let [x] denote the greatest integer less than or equal to x. (In other 
words, [x] is the integer left when you round x down, or truncate the 
fractional part.)

The number of leap years after 1600 and on or before the year N is 
given by the number of ordinary leap years [(N-1600)/4] = [N/4] - 400, 
less the number of centurial years [(N-1600)/100] = [N/100] - 16, plus 
the number of centurial leap years [(N-1600)/400] = [N/400] - 4, or:

     T = [N/4] - [N/100] + [N/400] - 388

If we write the year N = 100*C + D, where 0 <= D < 100, then this 
takes the form:

     T = 24*C + [C/4] + [D/4] - 388

Since on leap year the extra day is added at the end of February, we 
will consider the years to begin on March 1, and consider March to be 
the first month of year N, December the 10th month of year N, and the 
following January and February the 11th and 12th months of year N 
(although the calendar tells us they are in year N+1).

Now the number of days from March 1, 1600 to March 1 of the year 
100*C + D is given by:

     365*(100*(C-16)+D) + T

There remains the adjustment for the day and month to be made. If the 
number of the month using the above scheme is M, then we need to add 
30*(M-1) + [(3*M-1)/5] to find the number of days from March 1 to the 
first of the month with number M. Then to get from the first of the 
month to the current date X, add X-1.

Thus the number of days from March 1, 1600 to day X of month M of year
100*C + D is given by:

     Z = 365*(100*(C-16)+D) + 24*C + [C/4] + [D/4]
          + 30*M + [(3*M-1)/5] + X - 419

Trying this with April 3, 1601: C = 16, D = 1, M = 2, and X = 3, so

     Z = 365*(100*(16-16)+1) + 24*16 + [16/4] + [1/4]
          + 30*2 + [(3*2-1)/5] + 3 - 419

       = 365 + 0 + 4 + 384 + 60 + 1 + 3 - 419

       = 398 days

       = 365 + 31 + 2 days

which checks.

Now compute the Z values for one's date of birth and for the current 
date, and subtract one from the other. That will give you one's age in 
days.

For example, if one were born October 9, 1975, the Z value for that 
day is Z = 137,187. Today is May 31, 2000, and its Z value is Z = 
146,188. The difference is 9001 days, the exact age of this person.

Now converting the age in days to an age in years, months, and days is 
a little bit trickier, and an ambiguity of as much as two days can 
occur.

- Doctor Rob, The Math Forum
  http://mathforum.org/dr.math/