Not all conversion specifiers may be supported by your C library, in which case they will not be supported by PHP's strftime(). Additionally, not all platforms support negative timestamps, so your date range may be limited to no earlier than the Unix epoch. This means that %e, %T, %R and, %D (and possibly others) - as well as dates prior to Jan 1, 1970 - will not work on Windows, some Linux distributions, and a few other operating systems.
Developing this Flask app on Windows?
Good luck with that.
Python on Windows:
From http://php.net/manual/en/function.strftime.php:
More info from MSDN: http://msdn.microsoft.com/en-us/library/fe06s4ak(v=vs.110).aspx