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import datetime
# make datetime from string
start_time = datetime.datetime.strptime("2019-08-20 14:35:25", '%Y-%m-%d %H:%M:%S')
# datetime to string and print
print(start_time.strftime('%Y-%m-%d %H:%M:%S'))
# change datetime with given delta
start_time = start_time + datetime.timedelta(seconds=1)
print(start_time.strftime('%Y-%m-%d %H:%M:%S'))