ENH: ewm() default to using DatetimeIndex as times when halflife is a timedelta

[gsiano]

Allow ewm() to use a DatetimeIndex as the times parameter if it isn't provided and halflife is a timedelta convertible.

Before:

>>> now = pd.Timestamp.now()
>>> idx = [now + pd.Timedelta(seconds=i) for i in range(1, 4)]
>>> df = pd.DataFrame({'a': [1,2,3]}, index=idx)
>>> df.ewm(halflife=pd.Timedelta(seconds=1)).mean()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/gsiano/.local/lib/python3.10/site-packages/pandas/core/generic.py", line 11743, in ewm
    return ExponentialMovingWindow(
  File "/home/gsiano/.local/lib/python3.10/site-packages/pandas/core/window/ewm.py", line 386, in __init__
    raise ValueError(
ValueError: halflife can only be a timedelta convertible argument if times is not None.
>>> df.ewm(halflife=pd.Timedelta(seconds=1), times=df.index).mean()
                                   a
2023-07-18 10:45:18.007264  1.000000
2023-07-18 10:45:19.007264  1.666667
2023-07-18 10:45:20.007264  2.428571
>>> 

After:

>>> now = pd.Timestamp.now()
>>> idx = [now + pd.Timedelta(seconds=i) for i in range(1, 4)]
>>> df = pd.DataFrame({'a': [1,2,3]}, index=idx)
>>> df.ewm(halflife=pd.Timedelta(seconds=1)).mean()
                                   a
2023-07-18 10:46:46.589891  1.000000
2023-07-18 10:46:47.589891  1.666667
2023-07-18 10:46:48.589891  2.428571

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

This would be an API behavior change so this isn't possible as is. Was this discussed in an issue?

[gsiano]

This would be an API behavior change so this isn't possible as is. Was this discussed in an issue?

No, it wasn't. Should I create an issue? Or is something like this not worth an API change?

[mroeschke]

You could create an issue, but IMO this behavior is less explicit so IMO I am not sure this is a good change

[mroeschke]

Thanks for opening up #54187, going to close this PR for now until there has been more opt-in from other core developers in the issue