opptyId field   oldValue    newValue    updateTime
0   Stage       Qualify 2014-05-27T18:50:14
0   Forecast        Best Case   2014-05-27T18:50:14
0   created         2014-05-27T18:50:14
0   Amount      795.53  2014-06-17T18:54:00
0   Stage   Qualify Closed - Won    2014-07-09T20:11:05
0   Forecast    Best Case   Closed  2014-07-09T20:11:05
1   Stage       Target  2013-08-13T04:36:09
1   Forecast        Omitted 2013-08-13T04:36:09
1   created         2013-08-13T04:36:09
1   Amount      395 2013-08-15T03:51:15
1   Amount  395 1005726.5   2013-08-20T03:31:03
1   Amount  1005726.5   1068513.5   2013-08-29T04:51:30
1   Stage   Target  Closed - Won    2013-09-19T23:43:46
1   Forecast    Omitted Closed  2013-09-19T23:43:46

df['diff'] = df.sort_values(['opptyId','updateTime']).groupby('opptyId')['updateTime'].diff()