In addition to %d there are also other items that support single digit though zero padding is mentioned with strptime as below in the context of strftime : 

>>> import datetime
>>> datetime.datetime.strptime("1/1/2018 1:1:1", "%d/%m/%Y %I:%M:%S")
datetime.datetime(2018, 1, 1, 1, 1, 1)


>>> datetime.datetime.strftime(datetime.datetime(year=2018, month=1, day=1, hour=1, second=1, minute=1), "%d/%m/%Y %I:%M:%S")
'01/01/2018 01:01:01'

I couldn't find exact set of words that can be used to denote that zero padding is optional in strptime and it's returned as zero padded in strftime but if we are changing %d then I propose changing other items too with similar wording.

Thanks