Why do I get the wrong date back when I format a tsibble yearweek object back to year month day (yearweek "2020 W01" formats to "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"

^{Created on 2021-05-12 by the reprex package (v2.0.0)}

Answer 1

Because it operates as documented. Using the fine documentation in base R under help(strptime):

 ‘%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.      

Which we can then test using %Y-W following by the option:

> 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"                                
> 

What you get in your example are essentially just wrappers around the same functionality. Maybe yearweek() gives you a choice of convention; if not you now know how to build your own under a different option.