将非标准日期格式字符串（“April-20”）转换为日期对象 R

Question

I have a vector of date strings in the form month_name-2_digit_year ie

a = rbind("April-21", "March-21", "February-21", "January-21") 

I'm trying to convert that vector into a vector of date objects. I'm aware this question is very similar to this: Convert non-standard date format to date in R posted some years ago, but unfortunately, it has not answered my question.

I have tried the following as.Date() calls to do this, but it just returns a vector of NA. Ie

b = as.Date(a, format = "%B-%y")
b = as.Date(a, format = "%B%y")
b = as.Date(a, "%B-%y")
b = as.Date(a, "%B%y")

I'm also attempted to do it using the convertToDate function from the openxlsx package:

b = convertToDate(a, format = "%B-%y") 

I have also tried all the above but using a single character string rather than a vector, but that produced the same issue.

I'm a little lost as to why this isn't working, as this format has worked in reverse earlier in my script (that is, I had a date object already in dd-mm-yyyy format and converted it to month_name-yy using %B-%y). Is there another way to go from string to date when the string is a non-standard (anything other than dd-mm-yyy or mm-dd-yy if you're in the US) date format?

For the record my R locales are all UK and english.

Thanks in advance.

Answer 1

A Date must have all three of day, month and year. Convert to yearmon class which requires only month and year and then to Date as in (1) and (2) below or add the day as in (3).

(1) and (3) give first of month and (2) gives the end of the month.

(3) uses only functions from base R.

Also consider not converting to Date at all but just use yearmon objects instead since they directly represent a year and month which is what the input represents.

library(zoo)

# test input
a <- c("April-21", "March-21", "February-21", "January-21") 

# 1
as.Date(as.yearmon(a, "%B-%y"))
## [1] "2021-04-01" "2021-03-01" "2021-02-01" "2021-01-01"

# 2
as.Date(as.yearmon(a, "%B-%y"), frac = 1)
## [1] "2021-04-30" "2021-03-31" "2021-02-28" "2021-01-31"

# 3
as.Date(paste(1, a), "%d %B-%y")
## [1] "2021-04-01" "2021-03-01" "2021-02-01" "2021-01-01"

Answer 2

In addition to zoo, which @G. Grothendieck mentioned, you can also use clock or lubridate.

clock supports a variable precision calendar type called year_month_day . In this case you'd want "month" precision, then you can set the day to whatever you'd like and convert back to Date.

library(clock)

x <- c("April-21", "March-21", "February-21", "January-21") 

ymd <- year_month_day_parse(x, format = "%B-%y", precision = "month")
ymd
#> <year_month_day<month>[4]>
#> [1] "2021-04" "2021-03" "2021-02" "2021-01"

# First of month
as.Date(set_day(ymd, 1))
#> [1] "2021-04-01" "2021-03-01" "2021-02-01" "2021-01-01"

# End of month
as.Date(set_day(ymd, "last"))
#> [1] "2021-04-30" "2021-03-31" "2021-02-28" "2021-01-31"

The simplest solution may be to use lubridate::my() , which parses strings in the order of "month then year". That assumes that you want the first day of the month, which may or may not be correct for you.

library(lubridate)

x <- c("April-21", "March-21", "February-21", "January-21") 

# Assumes first of month
my(x)
#> [1] "2021-04-01" "2021-03-01" "2021-02-01" "2021-01-01"