r - Get date difference in years (floating point)

Question

I want to correct source activity based on the difference between reference and measurement date and source half life (measured in years). Say I have

ref_date <- as.Date('06/01/08',format='%d/%m/%y')

and a column in my data.frame with the same date format, e.g.,

today <- as.Date(Sys.Date(), format='%d/%m/%y')

I can find the number of years between these dates using the lubridate package

year(today)-year(ref_date)
[1] 5

Is there a function I can use to get a floating point answer today - ref_date = 5.2y, for example?

Answer 1

Yes, of course, use difftime() with an as numeric:

R> as.numeric(difftime(as.Date("2003-04-05"), as.Date("2001-01-01"), 
+                      unit="weeks"))/52.25
[1] 2.2529
R> 

Note that we do have to switch to weeks scaled by 52.25 as there is a bit of ambiguity there in terms of counting years---a February 29 comes around every 4 years but not every 100th etc.

So you have to define that. difftime() handles all time units up to weeks. Months cannot be done for the same reason of the non-constant 'numerator'.