Exactly as @Werner Stinckens​ said. Additionally you can share your file and script so we can help better.

Your screenshot looks like excel. If it is excel format please check is all fields a data format (you can change also to number as every date is number of days from 31st December 1899). If it is csv format as werners said you need to specify format because for example 6/5/2021 can crush as it can be 5th June or 6th May.