Python 如何使用与指定时间戳关联的值填充新列？

我的df看起来像这样

Date Time,Raw RMY Pad 7,RMY Pad 7 previous,difference,RMY Pad 7
2020-05-15 00:15:00,0.028,Nan,Nan,Nan
2020-05-16 00:15:00,0.028,0.028,0.0,0.028
2020-05-17 00:15:00,0.030,0.028,0.002,0.030
2020-05-18 00:15:00,0.050,0.030,0.02,0.050

我想添加一行，并用我指定的“日期-时间”值填充它。例如，如果我要具体说明

start_date = '2020-05-15'

start_date = '2020-05-17'

新列将填充0.028。如果我要具体说明

start_date = '2020-05-15'

start_date = '2020-05-17'

新列将填充0.030。我希望根据所选的开始日期将数据帧归零。所以基本上是

zeroed RMY pad 7 = start_date 'value' - baseline 'value'

我有一些数据帧，我正在比较，我想使用它

最后，我想要以下df

start_date = '2020-05-15'

Date Time,Raw RMY Pad 7,RMY Pad 7 previous,difference,RMY Pad 7,baseline,zeroed RMY pad 7
2020-05-15 00:15:00,0.028,Nan,Nan,Nan,Nan,Nan
2020-05-16 00:15:00,0.028,0.028,0.0,0.028,0.028,0.0
2020-05-17 00:15:00,0.030,0.028,0.002,0.030,0.028,0.002
2020-05-18 00:15:00,0.050,0.030,0.02,0.050,0.028,0.022

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要获取给定日期的基线值，应将索引设置为“日期-时间”列，然后使用。我们仅使用日期部分进行比较

df['zeroed RMY pad 7'] = df['Raw RMY Pad 7'] - df.set_index(df['Date Time'].dt.date).loc[pd.to_datetime(start_date).date(),'Raw RMY Pad 7']

开始日期的结果='2020-05-15'：