Formatting a Date Column with mixed data type

Given your displayed data,

you merely need to specify the culture as one that has a format of dd-mm-yyyy. It would appear that your computer's default date format is MDY and the mismatched values are in DMY format.

eg:

let

//replace Source line with your actual data source
    Source = Excel.CurrentWorkbook(){[Name="Table5"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"issue_date", type date}},"en-150")
in
    #"Changed Type"

"en-150" is the culture code for Europe, although others will work as well.

Note that the dates above are in MDY format which relates to my Windows Regional Settings.