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rlubridate
nacnudus
nacnudus

Reputation: 6538

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

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")

Upvotes: 34

Views: 34596

Answers (5)

Ma&#235;l
Ma&#235;l

Reputation: 52399

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"

Upvotes: 2

iauhs
iauhs

Reputation: 53

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)

Upvotes: 2

Frank
Frank

Reputation: 2416

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"

Upvotes: 76

Roland
Roland

Reputation: 132989

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"

Upvotes: 31

nacnudus
nacnudus

Reputation: 6538

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

[1] "2013-01-01 UTC"

Upvotes: 15

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