This bug occurs only when you use threads and only once.
Of course, this method is thread safe, but there is a severe warning. The first use of strptime is not thread secure (underneath _strptime is imported, and the import may throw AttributeError). If you want to avoid this problem, either you have to call strptime or import _strptime before starting a thread.
How to fix it?
Just import _strptime or call strptime before starting a thread.