Previous month datetime pandas

You can use DateOffset:

In [32]:    
dtDate = dt.datetime(2016,1,1,0,0)
dtDate - pd.DateOffset(months=1)

Out[32]:
Timestamp('2015-12-01 00:00:00')

You can use:

dtDate = datetime.datetime(2016,1,1,0,0)

print (dtDate - pd.DateOffset(months=1))
2015-12-01 00:00:00

print (dtDate - pd.DateOffset(years=1))
2015-01-01 00:00:00

Add s is important, because if use year only:

print (dtDate - pd.DateOffset(year=1))
0001-01-01 00:00:00