Python timedelta object with negative values

Question

I don't quite understand how negative arguments in datetime.timedelta are interpreted.

With Positive values:

>>> from datetime import timedelta
>>> d = timedelta(days=1,seconds=1,microseconds=1,milliseconds=1,minutes=1,hours=1,weeks=1)
>>> (d.days, d.seconds, d.microseconds)
>>> (8, 3661, 1001)

This is pretty straightforward. A similar example with negative values looks like:

>>> from datetime import timedelta
>>> d = timedelta(days=-1,seconds=-1,microseconds=-1,milliseconds=-1,minutes=-1,hours=-1,weeks=-1)
>>> (d.days, d.seconds, d.microseconds)
>>> (-9, 82738, 998999) 

As per my understanding seconds and microseconds are derived like:

• seconds = 86399 - (-60-3600-1)
• microseconds = 999999 - (-1-1000)

Is this correct? How come days equals -9?

I am reading this section of docs. But still don't quite understand the working with negative values. Please share explanations or relevant documentation links. Thanks :)

Answer 1

Because of the way timedeltas are stored internally, only the days attribute can take on negative values. This can be surprising when the timedelta is printed back. An illuminating example from the docs,

>>> d = timedelta(microseconds=-1)
>>> (d.days, d.seconds, d.microseconds)
(-1, 86399, 999999)

ie. -1d + 86399s + 999999µs = -1µs

Answer 2

It makes complete sense, (-1 week + -1 day) + (-1 hours) + (-1 minutes) + (-1 seconds) + (-1 milliseconds) + (-1 microseconds) equals to: (-8 days) + (-1 hours) + (-1 minutes) + (-1 seconds) + (-1 milliseconds) + (-1 microseconds)

By having less than -8 days, with -1 hours, -1 minutes, ... the number of days will have to be even less than -8 to make the negative hours, minutes, seconds... into positive hours, minutes, seconds....(since only days can be represented negatively, others,such as seconds, are always represented positively). Which means that days will have to be -9.

If you print d , you will get -9 days 22:58:58.998999, with negative 9 days and positive 22+ hours. Seeing the str of the timedelta could help you have better understanding of how negative timedelta is represented.