When I use numFmt = "DATE" in openxlsx, why does Excel store it as Custom format and not as Date?

If I run code from the openxlsx vignette, pasted below, and open the file, it displays correctly but Excel considers it a Custom format. Can anyone explain why and how to get Excel to recognize it as a Date datatype?

# data.frame of dates
dates <- data.frame(d1 = Sys.Date() - 0:4)
for (i in 1:3) dates <- cbind(dates, dates)
names(dates) <- paste0("d", 1:8)

## Date Formatting
wb <- createWorkbook()
addWorksheet(wb, "Date Formatting", gridLines = FALSE)
writeData(wb, 1, dates)  ## write without styling

## numFmt == 'DATE' will use the date format specified by the above
addStyle(wb, 1, style = createStyle(numFmt = "DATE"), rows = 2:11, cols = 1, gridExpand = TRUE)

saveWorkbook(wb, "Date Formatting.xlsx", overwrite = TRUE)