You can use the following methods to get the first or last day of the month for some date in R using functions from the package:

Method 1: Get First Day of Month

library(lubridate)df$first_day <- floor_date(ymd(df$date), 'month')

Method 2: Get Last Day of Month

library(lubridate)

df$last_day <- ceiling_date(ymd(df$date), 'month') - days(1)

The following examples show how to use each method in practice with the following data frame:

#create data frame
df <- data.frame(date=c('2022-01-05', '2022-02-18', '2022-03-21',
                        '2022-09-15', '2022-10-30', '2022-12-25'),
                 sales=c(14, 29, 25, 23, 39, 46))

#view data frame
df

        date sales
1 2022-01-05    14
2 2022-02-18    29
3 2022-03-21    25
4 2022-09-15    23
5 2022-10-30    39
6 2022-12-25    46

Example 1: Get First Day of Month Using lubridate

The following code shows how to use the floor_date() function from the lubridate package to get the first day of the month for each value in the date column:

#add new column that contains first day of month
df$first_day <- floor_date(ymd(df$date), 'month')

#view updated data frame
df

        date sales  first_day
1 2022-01-05    14 2022-01-01
2 2022-02-18    29 2022-02-01
3 2022-03-21    25 2022-03-01
4 2022-09-15    23 2022-09-01
5 2022-10-30    39 2022-10-01
6 2022-12-25    46 2022-12-01

Notice that the values in the new first_day column contain the first day of the month for each value in the date column.

Note: We used the ymd() function to first convert the strings in the date column to a recognizable date format.

Example 2: Get Last Day of Month Using lubridate

The following code shows how to use the ceiling_date() function from the lubridate package to get the last day of the month for each value in the date column:

#add new column that contains last day of month
df$last_day <- ceiling_date(ymd(df$date), 'month') - days(1)

#view updated data frame
df

        date sales   last_day
1 2022-01-05    14 2022-01-31
2 2022-02-18    29 2022-02-28
3 2022-03-21    25 2022-03-31
4 2022-09-15    23 2022-09-30
5 2022-10-30    39 2022-10-31
6 2022-12-25    46 2022-12-3122-12-01

Notice that the values in the new last_day column contain the last day of the month for each value in the date column.

Refer to the lubridate for more date formatting options.