How to subtract years?

The easiest thing to do is to convert it into POSIXlt and subtract 2 from the years slot.

> d <- as.POSIXlt(as.Date('2010/03/17'))
> d$year <- d$year-2
> as.Date(d)
[1] "2008-03-17"

See this related question: How to subtract days in R?.

With lubridate

library(lubridate)
ymd("2010/03/17") - years(2)

You could use seq:

R> dt = as.Date('2010/03/17')
R> seq(dt, length=2, by="-2 years")[2]
[1] "2008-03-17"

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