Extract only quarter from a date in r

There is a base R function, quarters, that more or less accomplishes what you want, though it prepends "Q". So

quarters(as.Date("2001-05-01"))
[1] "Q2"

If it is important to get rid of the "Q", you could use substr

substr(quarters(as.Date("2001-05-01")), 2, 2)
[1] "2"

Other date-related base R functions, such as weekdays and months can be found in help page ?quarters.

I would do:

# example data
DT = data.table(id = 1:10, d = as.IDate("2003-02-08") + seq(100, by=50, length.out=10))

DT[, qtr := quarter(d)]

    id          d qtr
 1:  1 2003-05-19   2
 2:  2 2003-07-08   3
 3:  3 2003-08-27   3
 4:  4 2003-10-16   4
 5:  5 2003-12-05   4
 6:  6 2004-01-24   1
 7:  7 2004-03-14   1
 8:  8 2004-05-03   2
 9:  9 2004-06-22   2
10: 10 2004-08-11   3

The quarter function is provided by data.table and works on both Date and IDate vectors. (IDate uses integer storage.)

lubridate package has the same function. We can use that also. I am using @Frank's DT

DT[, qtr := lubridate::quarter(d)]