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       Waiting2016-04-29 18:38:08         2016-04-29        -1 days +05:21:52

I wonder why these negative values appear ??

@cᴏʟᴅsᴘᴇᴇᴅ 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()