python - why I have negative date by subtraction of two column?

Question

I'm trying to create a column his values is the subtraction of two column but I found strange values:

 Patient["Waiting"] = Patient["Appointment"] - Patient["Scheduled"]

 Scheduled                   Appointment       Waiting
 2016-04-29 18:38:08         2016-04-29        -1 days +05:21:52

I wonder why these negative values appear ??

Answer 1

@c???s???? explain it better:

When two datetime objects are subtracted, the result is a timedelta. Depending on which date was larger, the result could be positive or negative.

Also if all values in column have no times, in pandas are not shown.

Patient["Waiting"] = Patient["Appointment"] - Patient["Scheduled"]
                     2016-04-29 00:00:00    - 2016-04-29 18:38:08 

For remove negative timedelatas is possible use abs:

Patient["Waiting"] = (Patient["Appointment"] - Patient["Scheduled"]).abs()