Рекомендовать пакет python-dateutil
python-dateutil
pip install python-dateutil
s1 = "October 1, 2018"
s2 = "23/05/2018"
s3 = "20-02-2017"
s4 = "May 2016"
from dateutil import parser
r1 = parser.parse(s1)
r2 = parser.parse(s2)
r3 = parser.parse(s3)
r4 = parser.parse(s4) # parser.parse(s4).timestamp() you can get timestamp this way
print(r1, r2, r3, r4)
# 2018-10-01 00:00:00
# 2018-05-23 00:00:00
# 2017-02-20 00:00:00
# 2016-05-04 00:00:00