Using lubridate

The lubridate package makes working with dates in R much easier. It has a number of helper functions for manipulating and extracting info from Date and POSIXct objects.

To get the first day of the current month, you can use floor_date() and pass it the current date:

library(lubridate)

today <- Sys.Date() # or Sys.time() for POSIXct
first_day <- floor_date(today, unit = "month")
first_day

[1] "2024-02-01"

This will return a Date object with the first day of the month.

To get the last day, use ceiling_date() instead:

last_day <- ceiling_date(today, unit = "month") - days(1)
last_day

[1] "2024-02-29"

You can also pass any Date object to these functions to get the first or last day of that month:

date <- ymd("2023-06-15")
floor_date(date, "month") # 2023-06-01

[1] "2023-06-01"

ceiling_date(date, "month") - days(1) # 2023-06-30

[1] "2023-06-30"

The lubridate functions make this really easy!

Base R Methods

You can also get the first and last day of month using just base R functions.

For the first day, use as.Date() with format() and pass it the year, month, and day 1:

first_day <- as.Date(format(today, "%Y-%m-01"))
first_day

[1] "2024-02-01"

For the last day, we can use 0 as the day which will give the last day of the month:

last_day <- as.Date((format(today + months(1), "%Y-%m-01")))-1
last_day

[1] "2024-02-29"

A bit more work than lubridate, but good to know you can do this with just base R.

I hope this helps you easily get the first and last day of the month in your own date analyses in R! Let me know if you have any other questions.