Why I Have Negative Date By Subtraction Of Two Column?

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'] Schedule

Solution 1:

@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()