This happens to me all the time. If I set a start date in format yyyy-mm-dd, and then for the end date use a formula such as startDate + days(5), the result always includes time. Luckily, upon insert, mySQL truncates the values to just the date.
Here's an example:
https://mockaroo.com/54dc8f60
If you look at the schema, I set the date for effectiveOn and format it yyyy-mm-dd, then calculate confirmedOn by subtracting 10 days. If you hit preview, you can see the result, which always includes the time in the calculated field.