BUG: `DatetimeIndex.is_year_start` returns incorrect array if `.freq == 'MS'`

[rwijtvliet]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd

i = pd.date_range('2020-01-01', freq='MS', periods=12)
# DatetimeIndex(['2020-01-01', '2020-02-01', '2020-03-01', '2020-04-01',
#                '2020-05-01', '2020-06-01', '2020-07-01', '2020-08-01',
#                '2020-09-01', '2020-10-01', '2020-11-01', '2020-12-01'],
#               dtype='datetime64[ns]', freq='MS')

# Incorrect result:
i.is_year_start 
# array([False, False, False, False, False, False, False, False, False, False, False, True])

# Correct result when first removing the frequency:
i.freq = None
i.is_year_start
# array([True, False, False, False, False, False, False, False, False, False, False, False])

Issue Description

The .freq property of the DatetimeIndex influences the result of the .is_year_start property. If it is set to MonthStart, the december timestamp is incorrectly recognised as the start of the year.

.is_quarter_start has a similar issue.

The issue does not arise for .freq == 'QS'.

Expected Behavior

Even if i.freq == 'MS', the result of i.is_year_start should start with True, not end with True.

Installed Versions

INSTALLED VERSIONS

commit : 91111fd
python : 3.9.13.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19045
machine : AMD64
processor : Intel64 Family 6 Model 165 Stepping 2, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : de_DE.cp1252

pandas : 1.5.1
numpy : 1.23.4
pytz : 2022.6
dateutil : 2.8.2
setuptools : 63.4.1
pip : 22.2.2
Cython : None
pytest : 7.1.3
hypothesis : None
sphinx : 5.3.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.5.0
pandas_datareader: None
bs4 : 4.11.1
bottleneck : None
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.6.2
numba : None
numexpr : None
odfpy : None
openpyxl : 3.0.10
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : None
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
zstandard : None
tzdata : None

[alexanderwertman3]

I can take this one

[lohmanndouglas]

• I have confirmed this bug exists on the main branch of pandas.

---

@jbrockmendel

I was investigating this issue, as I understood there is something strange in the function get_start_end_field.

[strange]
If freq == 'MS':
month_kw == 12 (default)
start_month == 12
end_month == 11

[looks ok]
If freq == 'QS':
month_kw == 1
start_month == 1
end_month == 12

[looks ok]
If freq == None:
month_kw == 12 (default - doesn't matter)
start_month == 1
end_month == 12

Needs help to investigate:

We set a month_kw? Why is 12?

I appreciate any help that guides me to figure out more about this issue :)

---

Update:

I found that in arrays/datetimes.py we are setting the month_kw to 12. I believe that here is the issue.

with QS (works why?)

In the scenario that we have a QS the freq object is QuarterBegin and this object has a startingMonth attribute, thus we set the month_kw to 1. It works!

with MS (does not work)

The freq object is MonthBegin that does not have the startingMonth, thus we set month_kw to 12.

But why its work with freq = None?

Because the function get_start_end_fieldcheck if freqstr is None. If it is None (line 261) its sets the start month to 1 again.

---

I will check the tests and certify that this is the right solution.

                if freq:
                    kwds = freq.kwds
-                  month_kw = kwds.get("startingMonth", kwds.get("month", 12))
+                  month_kw = kwds.get("startingMonth", kwds.get("month", 1))

---

@alexanderwertman3, are you working on it?

@jbrockmendel, @jorisvandenbossche May I open a PR with this solution?

The solution breaks some tests, and need to be improved. -> help wanted

[alexanderwertman3]

I am going to start investigating the issue this week. It does seem odd that month_kw is set to 12 in get_start_end_field when freq == 'MS'

[alexanderwertman3]

@lohmanndouglas:
From what I can understand, when freq == 'QS', the QuarterBegin object has a startingMonth attribute, so we correctly set month_kw to 1.
However, when freq == 'MS', the MonthBegin object does not have a startingMonth attribute, so we set month_kw to 12.

I am still looking into as to why this might be the case. Clearly, there is a reason for defaulting month_kw to 12 because the change you made is causing tests to fail.

[jbrockmendel]

Clearly, there is a reason for defaulting month_kw to 12 because the change you made is causing tests to fail.

which tests?