While it is unlikely that a purely numeric format such as "%Y-%m-%d
%H:%M:%S" will be locale dependent, it is possible that some pre-C99
systems would format dates using exotic digits is some locales.  Given
that format is so simple, I would just use explicit formatting of dt
components instead of datetime.strftime.  Doing so will also eliminate
a call to system strftime which is known to be quirky on popular
platforms even in C locale.