如何将值较少的日期时间列添加到单独的数据框中？

Question

我有一个带有两列的df，一列包含表示唯一字符的#，另一列包含日期时间值。我想将“date-time”值传递给一个单独的df，其中包含多个相同字符的条目

这就是df的样子。第二个df应该在最右边有“Timestamp”列，在复制和粘贴之后显示为diff

   char          timestamp
0     2  12/03/08 10:41:47
1     7  01/15/08 21:47:09
2     9  01/01/08 11:02:20
3    10  07/06/08 17:04:02
4    19  01/07/08 03:01:19


         char   level  race  charclass            zone   guild          timestamp
10114236     2      18   Orc     Shaman     The Barrens       6  12/03/08 10:41:47
622587       7      54   Orc     Hunter         Feralas      -1  01/15/08 21:47:09
623200       7      54   Orc     Hunter  Un'Goro Crater      -1  01/15/08 21:56:54
623826       7      54   Orc     Hunter     The Barrens      -1  01/15/08 22:07:23
624439       7      54   Orc     Hunter        Badlands      -1  01/15/08 22:17:08

df = pd.read_csv('\dataframes\wowah_data.csv')

df = df.sort_values(['char', ' timestamp'])

activationday = df.groupby('char')[' timestamp'].min().reset_index()

df['firstday'] = pd.concat(activationday[' timestamp'])

print(df.head())

我得到以下错误

TypeError: first argument must be an iterable of pandas objects, you passed an object of type "Series"