Preparing migration file from a Salesforce export

I've exported my CSV files from Salesforce, but the dates didn't come across as yyyy-MM-dd, so I went into Documents in Windows, opened up the export folder and then opened up the first exported file in Excel and tried to reformat the date field and then save the file ready for importing into Zoho. But the file will not save as a CSV file with dates in yyyy-MM-dd format. I've tried several work arounds, but none worked. My salesforce records are very standard and under 500 in number. Any help you can give would be appreciated. Thanks, Duncan