Using Created and Updated Date for Tickets API

I'm importing some tickets from a legacy ticketing system and I'm trying to pass in the original created and updated dates on the imported tickets. Also, I'm running a Python script to do the import, pulling from an Excel document. The Created and Updated dates are stored as such below in Excel. 

I pull these into my Python script and convert to a string for both 'created_at' and 'updated_at' in my API call, but it's putting in today's date/time for both values by default instead of the values from my spreadsheet. 

Any thoughts here? It's probably a conversion that needs to take place, but I can't figure it out.