Index      ID   V1  V2
00:00:00   K1   3   4
00:00:00   K2   4   3
00:00:00   K17  2   5
00:00:00   K11  8   6

df.index = pd.to_datetime(df.index)

Index                 ID   V1  V2
2019-02-18 00:00:00   K1   3   4
2019-02-18 00:00:00   K2   4   3
2019-02-18 00:00:00   K17  2   5
2019-02-18 00:00:00   K11  8   6

df.index = pd.to_timedelta(df.index) + pd.Timestamp('2015-01-20')
print (df)
             ID  V1  V2
Index
2015-01-20   K1   3   4
2015-01-20   K2   4   3
2015-01-20  K17   2   5
2015-01-20  K11   8   6