diff --git a/data-raw/simulate-data.R b/data-raw/simulate-data.R
index 7cb489f..c7ebe4b 100644
--- a/data-raw/simulate-data.R
+++ b/data-raw/simulate-data.R
@@ -106,7 +106,7 @@ create_fake_drug_name <- function(atc) {
 }
 
 to_wwyy <- function(x) {
-  format(lubridate::as_date(x), format = "%W%y")
+  paste0(lubridate::isoweek(lubridate::as_date(x)), stringr::str_sub(lubridate::isoyear(lubridate::as_date(x)), -2))
 }
 
 to_yyyymmdd <- function(x) {