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")

I know one way to do it, hopefully there are some smarter ways out there. — nacnudus, May 12 '14 at 06:53

score 74 · Accepted Answer · edited May 06 '16 at 11:11

74

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"

edited May 06 '16 at 11:11

Sirko

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answered Nov 12 '15 at 01:01

Frank

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Could we remove the tz? – ay__ya Jan 11 '22 at 19:25
@ay__ya try `lubridate::as_date` – Frank Jan 12 '22 at 21:50

score 31 · Answer 2 · answered May 12 '14 at 07:15

31

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"

answered May 12 '14 at 07:15

Roland

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+1 for not using a string formatting hack (like me). – nacnudus May 12 '14 at 19:42
Just came across this, nice way of going about it. Makes it really easy, was over complicating it also with lubridate. – Trevor Nederlof Dec 19 '14 at 18:03
This is also significantly faster. – Stan Apr 14 '21 at 15:54

score 15 · Answer 3 · answered May 12 '14 at 06:51

15

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

[1] "2013-01-01 UTC"

answered May 12 '14 at 06:51

nacnudus

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score 2 · Answer 4 · edited Dec 27 '21 at 03:09

2

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)

edited Dec 27 '21 at 03:09

ah bon

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answered Oct 26 '16 at 13:42

iauhs

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Maël · Answer 5 · 2022-06-02T08:20:34.383

2

You can simply use base R's trunc:

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

edited Jun 02 '22 at 08:20

answered Apr 25 '22 at 19:19

Maël

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

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