It seems like your dates are in the wrong format. The dd (day-number) and mm (month) are swapped. Only dates with a dd <= 12 are (wrongly) converted, while months with a value > 12 are ignored and treated as text.
Do not open the csv file in a normal way, but import it with the Text Import Wizard (Excel 2016):
- Create a new file and goto Data -> Get External Date -> From Text
- Import the csv file
- In step 2 choose the correct delimiter (probably Comma or Semicolon)
- In step 3 select the first column in the Data preview and choose Date with the format MDY (not DMY!)
Now the dates will be converted correctly.
- Select the column with the dates
- Now go to the Format Cells window by clicking the small arrow button in the bottom-right corner of the Alingment area on the Home tab. Select the date format you want.
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