Quoting @Umair: “I have downloaded …”
You surely got a table contained in a plain text file regarding the CSV conventions basically. When “opening” such a file with LibO Calc you actually import and convert the contents. And you are prompted to choose the setting for the import/conversion process because there are many versions of “CSV” and in specific the suppliers of those CSV generally don’t regard a reasonable standard for dates. In your downloaded file a “date” will be a text looking like “7/12/17” e.g. which is ambiguous in a hazardous way. Calc must disambiguate this to get a date represented in the numeric way generally used by spreadsheets.
In the mentioned dialogue poping up:
-1- Enable ‘Detect special numbers’
-2- Select the column containing your dates by clicking on the header label.
-3- Open the list next to ‘Column type:’ and select the order of year, month, and date as contained in the actual data.
-4- ‘OK’.
For a column of dates already imported as text you get offered the needed tool via ‘Data’ > ‘Text to Columns…’. data.
Don’t forget to tell you bank they shall use the internationally approved and unambiguous date format YYYY-MM-DD (ISO8601) in the future to avoid such problems for their customers. Bank accounting data isn’t a letter to aunt Alice, after all!