Add 24 hours to a negative time difference in python

I am using python to calculate time intervals between two events. Each event has a 'beginning time' and an 'ending time.' I have found the difference between the two in a new column, 'interval', but have negative values when the beginning and ending time are on different days (for instance begin 23:46:00 and end 00:21:00 gives -23:25:00). I would like to create an if-statement to run through the 'interval' column and add 24 hours to any negative values. However, I have had problems with adding 24 hours to the 'interval' values. Currently my 'interval' dtype=timedelta64[ns].

Here is a little bit of the table to clarify the problem:

        CallDate      BeginningTime        EndingTime            Interval
    75  1/8/2009    1900-01-01 07:49:00  1900-01-01 08:19:00     00:30:00
    76  1/11/2009   1900-01-01 14:37:00  1900-01-01 14:59:00     00:22:00
    77  1/9/2009    1900-01-01 09:29:00  1900-01-01 09:56:00     00:27:00
    78  1/11/2009   1900-01-01 09:20:00  1900-01-01 10:13:00     00:53:00
    79  1/16/2009   1900-01-01 15:11:00  1900-01-01 15:50:00     00:39:00
    80  1/17/2009   1900-01-01 22:52:00  1900-01-01 23:26:00     00:34:00
    81  1/19/2009   1900-01-01 05:48:00  1900-01-01 06:32:00     00:44:00
    82  1/20/2009   1900-01-01 23:46:00  1900-01-01 00:21:00     -23:25:00
    83  1/20/2009   1900-01-01 21:29:00  1900-01-01 22:08:00     00:39:00
    84  1/23/2009   1900-01-01 07:33:00  1900-01-01 07:55:00     00:22:00
    85  1/30/2009   1900-01-01 19:33:00  1900-01-01 20:01:00     00:28:00