Dates in excel are always giving trouble :(. There are a few options I always try:
1: go back to source if you can and fix it there.
2: if you import it from excel as '* .csv' try reading from excel directly e.g. using readxl() or openxls() . Or, case you already read directly from the excel format, try other package or import as .csv by exporting from excel first (check char encoding is UTF-8)
3: if there are only a few different entries, just use dplyr::mutate(is.numeric()) to change them individually within R. Best to use lubridate when dealing with different formats.
Please note that for number conversion, the reference date is different when comparing excel and R. I think excel starts at 1900, so you need to use 'origin', e. g. as.Date(44405, origin = "1899-12-30") for today.