A call to
time.strftime('%')
returns
'%'

A similar call to
datetime.utcfromtimestamp(int(time.time()).strftime('%')
raises
ValueError: strftime format ends with raw %

Similar inputs like '%D %' behave similarly.

I might take a crack at fixing this, but first I wanted to see what the official guidance is. Seems to me like similar error handling behavior between the functions  would be desirable.