import numpy as np
import pandas as pd
ar = np.array([['2018-03-14T10:58:20.000Z', 2],
               ['2018-03-14T11:58:20.000Z', 3],
               ['2018-03-14T12:58:20.000Z', 2],
               ['2018-03-14T13:58:20.000Z', 10],
               ['2018-03-14T14:58:20.000Z', 11],
               ['2018-03-14T15:58:20.000Z', 12],
               ['2018-03-14T16:58:20.000Z', 11],
               ['2018-03-14T17:58:20.000Z', 3],
               ['2018-03-14T18:58:20.000Z', 2],
               ['2018-03-14T19:58:20.000Z', 4],
              ])
df = pd.DataFrame(ar, columns = ['Date', 'weight'])
df['Date'] = pd.to_datetime(df['Date'])
df['weight'] = df['weight'].astype(float)

df_new =      [['2018-03-14T10:58:20.000Z', 2],
               ['2018-03-14T11:58:20.000Z', 3],
               ['2018-03-14T12:58:20.000Z', 2],
               ['2018-03-14T13:58:20.000Z', 2],
               ['2018-03-14T14:58:20.000Z', 3],
               ['2018-03-14T15:58:20.000Z', 4],
               ['2018-03-14T16:58:20.000Z', 3],
               ['2018-03-14T17:58:20.000Z', 3],
               ['2018-03-14T18:58:20.000Z', 2],
               ['2018-03-14T19:58:20.000Z', 4],
              ])