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To calculate the week of a date, there are three functions in the lubridate package. They use different definitions for the start of the first week in the year. Please see the help section in lubridate for an explanation.
I used the mdy() function from lubridate to change the text strings into Dates.
library(lubridate)
#>
#> Attaching package: 'lubridate'
#> The following object is masked from 'package:base':
#>
#> date
library(dplyr)
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:lubridate':
#>
#> intersect, setdiff, union
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
DF <- data.frame(Date = mdy(c("9/1/2019",
"9/5/2019",
"9/11/2019",
"9/16/2019",
"9/20/2019",
"9/26/2019",
"9/30/2019")))
DF <- DF %>% mutate(Week = week(Date), ISOweek = isoweek(Date), EPIweek = epiweek(Date))
#> Warning: The `printer` argument is deprecated as of rlang 0.3.0.
#> This warning is displayed once per session.
DF
#> Date Week ISOweek EPIweek
#> 1 2019-09-01 35 35 36
#> 2 2019-09-05 36 36 36
#> 3 2019-09-11 37 37 37
#> 4 2019-09-16 37 38 38
#> 5 2019-09-20 38 38 38
#> 6 2019-09-26 39 39 39
#> 7 2019-09-30 39 40 40
Created on 2019-10-09 by the reprex package (v0.3.0.9000)