As this question started with this code to read excel file as text.
myFile <- lapply(sheets, function(X) readxl::read_excel(filename, sheet = X, col_names = T, skip = 5
, col_types = "text"
))
myFile is read as text
So, all the columns are read as "text" including dates (Format: 11-03-2019) column.
# The columns with date is **not read** as below:
`Sales of the Car produced in european (Date)` = c("2019-03-08", "2019-03-07"),
`Product designed and Contract signed date` = c("2018-03-08", "2018-03-07")
Instead
`Sales of the Car produced in european (Date)` = c("43427","43116"),
`Product designed and Contract signed date` = c("42124", "43427")
Now, when I used your code to convert to date format
myFile %>%
mutate_at(vars(matches("date|since")), ~dmy(.x))
those dates' columns have
only NA