How long until this date?
R, 99 characters

I know it is sort of cheating, but R is all about its packages and lubridate is so convenient for this kind of tasks!

f=function(x){library(lubridate);cat(show(as.period(interval(mdy("3/15/2012"),mdy(x)))),"until",x)}


Usage:

f("10/31/2017")
[1] 5 years, 7 months and 16 days 
5 years, 7 months and 16 days  until 10/31/2017