how R handles week starts: Sunday vs Monday start

week_numbers.R

library("lubridate")
#> 
#> Attaching package: 'lubridate'
#> The following object is masked from 'package:base':
#> 
#>     date

a_sunday <- as.Date("2017-08-27")

# floor_date rounds to Sunday:
weekdays(floor_date(Sys.Date(), "week"))
#> [1] "Sunday"

# week starts on Sunday:
lubridate::week(a_sunday - 1)
#> [1] 34
lubridate::week(a_sunday)
#> [1] 35

# isoweek starts on Monday:
lubridate::week(a_sunday)
#> [1] 35
lubridate::week(a_sunday + 1)
#> [1] 35

# format.Date: %U starts on Sunday
format(a_sunday - 1, "%U")
#> [1] "34"
format(a_sunday, "%U")
#> [1] "35"

# format.Date: %W starts on Monday
format(a_sunday, "%W")
#> [1] "34"
format(a_sunday + 1, "%W")
#> [1] "35"