[Solved] why I have negative date by subtraction of two column?

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@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()

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solved why I have negative date by subtraction of two column?