External System Import - how can I update a CSV to meet the requirements?

[Robert Horan]

I am trying to import data from another system and all of the dates are in the format 3/14/2019

So m/dd/yyyy. (or M/dd/yyyy)

Someone told me I need to add a 0 so it becomes 03/14/2019

If I had a couple of rows and a few columns that might be an option, but I have thousands of rows, dozens of columns, and multiple date and date/time fields.  Manual editing is not realistic.  

I cannot find a way to do this in Excel or Google Sheets.  Changing the format only affects the display of data, not the values.

I will listen to all options, though if a coding solution is required I might need to ask for a little more help.