First day of the month from a POSIXct date time using lubridate

Question

Given a POSIXct date time, how do you extract the first day of the month for aggregation?

library(lubridate)

full.date <- ymd_hms("2013-01-01 00:00:21")

Answer 1

lubridate has a function called floor_date which rounds date-times down. Calling it with unit = "month" does exactly what you want:

library(lubridate)
full.date <- ymd_hms("2013-01-01 00:00:21")
floor_date(full.date, "month")

[1] "2013-01-01 UTC"

Answer 2

I don't see a reason to use lubridate:

full.date <- as.POSIXct("2013-01-11 00:00:21", tz="GMT")

monthStart <- function(x) {
  x <- as.POSIXlt(x)
  x$mday <- 1
  as.Date(x)
}

monthStart(full.date)
#[1] "2013-01-01"

Answer 3

first.of.month <- ymd(format(full.date, "%Y-%m-01"))
first.of.month

[1] "2013-01-01 UTC"

Answer 4

i have another solution :

first.of.month <- full.date - mday(full.date) + 1

but it needs the library 'lubridate' or 'date.table' (aggregation with data.table)

Answer 5

You can also use base R's trunc:

d <- as.POSIXct("2013-01-11 00:00:21", tz="UTC")
trunc(d, "month")
#[1] "2013-01-01 UTC"