I'd like to find software (preferably free) that will help me compute
the interval between any two dates, in years, months, and days.

I don't want some code that I can add into my SQL program, and I don't
want intervals calculated on 360-day investment years or delivered in
raw number of days, no matter how many, nor do I want to supply data
in Julian form.  I just want to put in, say, a date of birth (month,
day, and year) and the date of an event, and get back the age of the
person at the time of the event; or the length of time between two
events; or the age of a person at the time of death, given birth date.
 In other words, date a and date b are x years, y months, and z days
apart.

And it should work with any year since the year 1 C.E.

Can you point me to a website or available software that will perform
this kind of computation for me?

Thank you,
Archae0pteryx

Archae0pteryx...

I believe the following webpage handily addresses your request.
TimeAndDate.com:
http://www.timeanddate.com/date/duration.html

When you're done with your calculation, you'll see a plethora
of other useful links at the bottom of the page, as well, or
you can just go to the homepage:
http://www.timeanddate.com/


You might also find the page that pointed me to that site to
be of some use, from the AARP website:
http://www.aarp.org/learntech/computers/software/free_calendars_to_organize_your_life.html

sublime1-ga


Searches done, via Google:

"how many days has it been"
://www.google.com/search?q=%22how+many+days+has+it+been%22

Thank you, sublime-1, that is exactly what I wanted.  I'm a little
chagrined because I use that website all the time to print out
calendars, not only for the current year but also for my chronologies
in the 14th century, and I never knew that function was there.  I'll
give it all a much closer look now.

Archae0pteryx

it even accounts for the skipped days when switching from the Julian
to the Gregorian calendars (not sure if archae0pteryx-ga is planning
on going back that far ;)
simply amazing..

Much further, actually, indistinguishable-ga.  I am working on a piece
of fiction that starts in 1306 and spans about a century and a half. 
I have to keep a grip on numerous characters' ages as various events
occur, and my weary fingers are a poor substitute for a date
calculator.

I like your handle.

Archae0pteryx