Batch date adjustment in CSV files

Hi. I need a simple script (or VBA code) that will scan a folder for CSV folders and take each file and adjust the dates in them by a number of days (-10000 to +10000). Dates are in [login to view URL] format. The original date format and the rest of the values in each line of the CSV must be kept intact. Program should be able to handle leap years (which if using program function should be easy to calculate new date). Script should be reasonably well optimized as file size can be around 350Mb. Don't use a language where I need to install heavy footprint to run the script / program! Excel is installed so can use VBA. For simplicity the script/ program will run from the same folder as the CSV files and use that as the default path. e.g. -10 days 2020.10.09 --> 2020.08.29