当我将 tsibble yearweek 对象格式化回年月日时，为什么我会得到错误的日期（年周“2020 W01”格式为“2020-12-30”）

Question

library(tsibble)
#> 
#> Attaching package: 'tsibble'
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, union

(yw <- yearweek("2020-01-01"))
#> <yearweek[1]>
#> [1] "2020 W01"
#> # Week starts on: Monday
format(yw, "%Y-%m-%d")
#> [1] "2020-12-30"

^{由reprex package (v2.0.0) 于 2021-05-12 创建}

Answer 1

因为它按照记录的方式运行。在help(strptime) 下使用base R 中的精细文档：

 ‘%U’ Week of the year as decimal number (00-53) using Sunday as    
      the first day 1 of the week (and typically with the first     
      Sunday of the year as day 1 of week 1).  The US convention.        

 ‘%V’ Week of the year as decimal number (01-53) as defined in ISO 
      8601.  If the week (starting on Monday) containing 1 January 
      has four or more days in the new year, then it is considered 
      week 1.  Otherwise, it is the last week of the previous year,
      and the next week is week 1.  (Accepted but ignored on       
      input.)                                                      
                                                                   
 [...]      

 ‘%W’ Week of the year as decimal number (00-53) using Monday as   
      the first day of week (and typically with the first Monday of
      the year as day 1 of week 1).  The UK convention.      

然后我们可以使用%Y-W 进行测试，然后选择：

> strftime(as.Date("2020-01-01"), "%Y-W%U")      
[1] "2020-W00"                                
> strftime(as.Date("2020-01-01"), "%Y-W%V")   
[1] "2020-W01"                                
> strftime(as.Date("2020-01-01"), "%Y-W%W")   
[1] "2020-W00"                                
> 

您在示例中得到的基本上只是相同功能的包装。也许yearweek() 给你一个约定的选择；如果不是，您现在知道如何在不同的选项下构建自己的。