import numpy as np
a = np.array([['2018-04-01T15:30:00'],
       ['2018-04-01T15:31:00'],
       ['2018-04-01T15:32:00'],
       ['2018-04-01T15:33:00'],
       ['2018-04-01T15:34:00']], dtype='datetime64[s]')
c = np.array([0,1,2,3,4]).reshape(-1,1)
c = c.astype("object")
d = np.append(c,a,axis=1)
d

array([[0, datetime.datetime(2018, 4, 1, 15, 30)],
       [1, datetime.datetime(2018, 4, 1, 15, 31)],
       [2, datetime.datetime(2018, 4, 1, 15, 32)],
       [3, datetime.datetime(2018, 4, 1, 15, 33)],
       [4, datetime.datetime(2018, 4, 1, 15, 34)]], dtype=object)