Quarter.onOffset looks fishy

[jbrockmendel]

qe = pd.offsets.QuarterEnd(startingMonth=2)
qs = pd.offsets.QuarterBegin(startingMonth=2)
bqe = pd.offsets.BQuarterEnd(startingMonth=2)
bqs = pd.offsets.BQuarterBegin(startingMonth=2)

feb1 = pd.Timestamp('2017-02-01')
feb28 = pd.Timestamp('2017-02-28')
apr30 = pd.Timestamp('2017-04-30')

>>> qs.onOffset(feb1)
True
>>> qe.onOffset(feb28)
True
>>> qe.onOffset(apr30)
False

>>> bqs.onOffset(feb1)
True
>>> bqe.onOffset(feb28)
True
>>> bqe.onOffset(apr30)
False

The QuarterStart behavior makes sense to me; the QuarterEnd does not. If Feb1 is the start of a quarter, shouldn't the end of that same quarter be Apr30?

[jbrockmendel]

On a closer look, it appears that the behavior is correct but misleading. QuarterEnd(startingMonth=2) is an offset for quarters that start on Feb 1, May 1, Aug 1, Nov 1. I assumed that QuarterEnd(startingMonth=2) would be an offset corresponding to the ends of those same quarters. That is incorrect. QuarterEnd(startingMonth=2) is an offset for quarters that end on Feb 2[89], May 31, Aug 31, Nov 30.

I'm not the first to make this mistake or otherwise end up confused:

• 5307 Parameter Name of startingMonth for QuarterEnd is Misleading

• 8435 DEPR: QuarterBegin and BQuarterBegin return days that are not quarter beginnings

• 17787 DOC: Anchored Offset Starting Quarter and Year defaults to JAN not DEC

• 2885 Default for "Q" is not the same default for QuarterEnd

I suggest the following:

• startingMonth should be replaced with something non-camelCase anyway, so let's take the opportunity to replace a problematic kwarg with a clear one.
• Just brainstorming here: q1start_month=2 means Q1 begins Feb1 and ends Apr 30 (\pm business days)
• Methods of the form to_qtr_end, to_qtr_business_begin, to_year_end (hopefully its obvious what they would do).

Thoughts? I'm happy to implement+test, but would like to get buy-in first.