As I have mentioned a time or two, show the code you have run. All of it at this point in time.
I have seen many different problems with Excel dates and the one you show almost certainly involves your code changing a value as read from Excel.
One thing to try is to make sure that entire columns of Excel have the same cell formatting, especially dates.
Save the file as CSV.
Then import the CSV file, use the option guessingrows=max or the number of lines in the file.
You are much more likely to get usable dates that way.