Implement scalar shift_month mirroring tslib.shift_months
replace relativedelta usage in relevant cases.

This should be orthogonal to other ongoing offsets PRs.

Ran asv repeatedly overnight, posting results below.
Hello @jbrockmendel! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on November 12, 2017 at 00:22 Hours UTC We would expect the Month, Quarter, Year (and Business/Custom) offsets to be affected by this:

```
taskset 4 asv continuous -f 1.1 -E virtualenv master HEAD -b timeseries -b period -b timedelta
[...]
       before           after         ratio
     [ca737aca]       [3a2dc531]
+         760±1ns          862±3ns     1.13  period.Properties.time_is_leap_year
+     6.36±0.01μs      7.15±0.01μs     1.12  timeseries.DatetimeIndex.time_timestamp_tzinfo_cons
-      16.7±0.1μs       15.0±0.1μs     0.90  timeseries.Offsets.time_custom_bday_apply
-     11.7±0.06μs      10.4±0.05μs     0.89  timeseries.Offsets.time_timeseries_year_incr
-       152±0.4μs          132±2μs     0.87  timeseries.Offsets.time_custom_bmonthbegin_incr_n
-         131±1μs        104±0.1μs     0.80  timeseries.Offsets.time_custom_bmonthbegin_decr_n
-       135±0.3μs         86.3±1μs     0.64  timeseries.Offsets.time_custom_bmonthend_incr
-       161±0.6μs        101±0.5μs     0.63  timeseries.Offsets.time_custom_bmonthend_decr_n
-       160±0.5μs       95.0±0.2μs     0.59  timeseries.Offsets.time_custom_bmonthend_incr_n
-      56.2±0.1μs      21.0±0.07μs     0.37  timeseries.SemiMonthOffset.time_end_decr_n
-      53.0±0.1μs      19.7±0.04μs     0.37  timeseries.SemiMonthOffset.time_end_incr_n
-      55.1±0.2μs      20.4±0.07μs     0.37  timeseries.SemiMonthOffset.time_begin_decr_n
-      52.1±0.1μs      19.2±0.07μs     0.37  timeseries.SemiMonthOffset.time_begin_incr_n
-     52.0±0.06μs      18.9±0.04μs     0.36  timeseries.SemiMonthOffset.time_begin_decr
-      46.8±0.1μs      16.0±0.05μs     0.34  timeseries.SemiMonthOffset.time_begin_incr
-      56.5±0.3μs      19.0±0.09μs     0.34  timeseries.SemiMonthOffset.time_end_decr
-     47.9±0.07μs      15.8±0.03μs     0.33  timeseries.SemiMonthOffset.time_end_incr
-      43.3±0.3μs      12.7±0.04μs     0.29  timeseries.SemiMonthOffset.time_begin_apply
-     45.1±0.08μs      12.0±0.05μs     0.27  timeseries.SemiMonthOffset.time_end_apply

taskset 4 asv continuous -f 1.1 -E virtualenv master HEAD -b timeseries -b period -b timedelta
[...]
       before           after         ratio
     [ca737aca]       [3a2dc531]
+        456±10ms         578±10ms     1.27  timeseries.ToDatetime.time_format_no_exact
+        27.1±2ms           31.8ms     1.17  timeseries.DatetimeIndex.time_dti_factorize
+      23.8±0.1μs      27.6±0.07μs     1.16  timeseries.Offsets.time_custom_bday_cal_incr_n
+      28.7±0.2μs       32.5±0.2μs     1.13  timeseries.Offsets.time_custom_bday_decr
+      15.2±0.2μs      16.9±0.08μs     1.12  timeseries.Offsets.time_custom_bday_apply
+        6.73±0μs      7.50±0.02μs     1.11  timeseries.DatetimeIndex.time_timestamp_tzinfo_cons
+           35.9s            39.7s     1.11  gil.nogil_datetime_fields.time_period_to_datetime
-       165±0.3μs        126±0.7μs     0.76  timeseries.Offsets.time_custom_bmonthbegin_incr_n
-         153±3μs         97.8±4μs     0.64  timeseries.Offsets.time_custom_bmonthend_incr_n
-       161±0.2μs        101±0.3μs     0.63  timeseries.Offsets.time_custom_bmonthend_decr_n
-       136±0.2μs       80.2±0.4μs     0.59  timeseries.Offsets.time_custom_bmonthend_incr
-      51.1±0.1μs      19.4±0.08μs     0.38  timeseries.SemiMonthOffset.time_begin_decr
-      55.7±0.5μs      20.6±0.07μs     0.37  timeseries.SemiMonthOffset.time_begin_decr_n
-      51.7±0.3μs       18.9±0.2μs     0.37  timeseries.SemiMonthOffset.time_end_incr_n
-        53.0±1μs       19.2±0.1μs     0.36  timeseries.SemiMonthOffset.time_end_decr
-      60.0±0.6μs      21.7±0.07μs     0.36  timeseries.SemiMonthOffset.time_end_decr_n
-     47.5±0.08μs      16.6±0.06μs     0.35  timeseries.SemiMonthOffset.time_begin_incr
-     53.9±0.06μs      18.7±0.08μs     0.35  timeseries.SemiMonthOffset.time_begin_incr_n
-      49.3±0.1μs      16.6±0.06μs     0.34  timeseries.SemiMonthOffset.time_end_incr
-      43.7±0.1μs      12.9±0.07μs     0.29  timeseries.SemiMonthOffset.time_begin_apply
-      43.8±0.1μs      12.1±0.04μs     0.28  timeseries.SemiMonthOffset.time_end_apply

taskset 4 asv continuous -f 1.1 -E virtualenv master HEAD -b timeseries -b period -b timedelta
[...]
       before           after         ratio
     [ca737aca]       [3a2dc531]
+        301±10ms          405±7ms     1.35  timeseries.AsOfDataFrame.time_asof
+       132±0.2ms        154±0.8ms     1.16  timeseries.ToDatetime.time_iso8601_tz_spaceformat
+     26.8±0.09μs       30.4±0.2μs     1.13  timeseries.Offsets.time_custom_bday_cal_incr_neg_n
-     3.78±0.01ms      3.43±0.01ms     0.91  period.Constructor.time_from_pydatetime
-     24.3±0.07μs       21.6±0.1μs     0.89  timeseries.AsOf.time_asof_single
-     7.24±0.02μs       6.41±0.2μs     0.89  timeseries.DatetimeIndex.time_timestamp_tzinfo_cons
-         904±5ns          790±3ns     0.87  period.Properties.time_quarter
-       154±0.5μs        127±0.3μs     0.83  timeseries.Offsets.time_custom_bmonthbegin_incr_n
-       129±0.3μs        104±0.6μs     0.81  timeseries.Offsets.time_custom_bmonthbegin_decr_n
-         567±2ms        459±0.8ms     0.81  timeseries.ToDatetime.time_format_exact
-       153±0.2μs         95.5±1μs     0.62  timeseries.Offsets.time_custom_bmonthend_incr_n
-       164±0.2μs        102±0.1μs     0.62  timeseries.Offsets.time_custom_bmonthend_decr_n
-       136±0.1μs         76.8±1μs     0.56  timeseries.Offsets.time_custom_bmonthend_incr
-     53.6±0.07μs       21.9±0.1μs     0.41  timeseries.SemiMonthOffset.time_begin_decr_n
-      51.0±0.1μs      19.6±0.04μs     0.38  timeseries.SemiMonthOffset.time_begin_decr
-     53.7±0.09μs      20.6±0.06μs     0.38  timeseries.SemiMonthOffset.time_end_decr_n
-      51.8±0.1μs       18.8±0.1μs     0.36  timeseries.SemiMonthOffset.time_end_decr
-      51.8±0.1μs      18.4±0.03μs     0.36  timeseries.SemiMonthOffset.time_end_incr_n
-     52.0±0.09μs      18.4±0.06μs     0.35  timeseries.SemiMonthOffset.time_begin_incr_n
-     47.4±0.06μs      15.6±0.04μs     0.33  timeseries.SemiMonthOffset.time_end_incr
-      47.7±0.3μs       15.7±0.1μs     0.33  timeseries.SemiMonthOffset.time_begin_incr
-     43.3±0.06μs      12.8±0.03μs     0.30  timeseries.SemiMonthOffset.time_begin_apply
-     45.4±0.03μs      12.3±0.03μs     0.27  timeseries.SemiMonthOffset.time_end_apply

taskset 4 asv continuous -f 1.1 -E virtualenv master HEAD -b timeseries -b period -b timedelta
[...]
       before           after         ratio
     [ca737aca]       [3a2dc531]
+         720±2ns          841±6ns     1.17  period.Properties.time_day
+           33.8s            39.3s     1.16  gil.nogil_datetime_fields.time_period_to_datetime
+      24.8±0.1μs       28.3±0.2μs     1.14  timeseries.Offsets.time_custom_bday_cal_incr
+         458±1ms          515±1ms     1.13  timeseries.ToDatetime.time_format_exact
+     27.4±0.08μs       30.1±0.2μs     1.10  timeseries.Offsets.time_custom_bday_cal_incr_neg_n
-         432±1ms          385±2ms     0.89  timeseries.SeriesArithmetic.time_add_offset_slow
-      28.6±0.2μs       24.9±0.1μs     0.87  timeseries.Offsets.time_custom_bday_cal_incr_n
-       149±0.3μs        128±0.2μs     0.86  timeseries.Offsets.time_custom_bmonthbegin_incr_n
-       129±0.4μs        109±0.2μs     0.85  timeseries.Offsets.time_custom_bmonthbegin_decr_n
-         879±4ns        732±0.7ns     0.83  period.PeriodProperties.time_hour
-       157±0.4μs         99.4±1μs     0.64  timeseries.Offsets.time_custom_bmonthend_decr_n
-       152±0.3μs       95.6±0.2μs     0.63  timeseries.Offsets.time_custom_bmonthend_incr_n
-       140±0.2μs       79.7±0.6μs     0.57  timeseries.Offsets.time_custom_bmonthend_incr
-      55.4±0.2μs      21.4±0.06μs     0.39  timeseries.SemiMonthOffset.time_begin_decr_n
-     51.6±0.07μs      19.8±0.02μs     0.38  timeseries.SemiMonthOffset.time_begin_incr_n
-     58.5±0.08μs      21.5±0.07μs     0.37  timeseries.SemiMonthOffset.time_end_decr_n
-      52.5±0.1μs      19.0±0.04μs     0.36  timeseries.SemiMonthOffset.time_begin_decr
-     51.9±0.07μs       18.6±0.1μs     0.36  timeseries.SemiMonthOffset.time_end_decr
-      53.6±0.1μs      18.2±0.07μs     0.34  timeseries.SemiMonthOffset.time_end_incr_n
-     48.6±0.05μs      15.6±0.03μs     0.32  timeseries.SemiMonthOffset.time_end_incr
-      50.0±0.1μs      15.4±0.04μs     0.31  timeseries.SemiMonthOffset.time_begin_incr
-     43.8±0.08μs      12.7±0.06μs     0.29  timeseries.SemiMonthOffset.time_end_apply
-      45.4±0.1μs      12.8±0.07μs     0.28  timeseries.SemiMonthOffset.time_begin_apply
-         290±4ms       27.2±0.2ms     0.09  timeseries.AsOfDataFrame.time_asof_nan
-         298±2ms       23.9±0.2ms     0.08  timeseries.AsOfDataFrame.time_asof

taskset 4 asv continuous -f 1.1 -E virtualenv master HEAD -b timeseries -b period -b timedelta
[...]
       before           after         ratio
     [ca737aca]       [3a2dc531]
+         745±7ns          978±4ns     1.31  period.Properties.time_second
+         762±2ns          956±3ns     1.26  period.Properties.time_is_leap_year
+         743±4ns          908±6ns     1.22  period.Properties.time_week
+      17.7±0.1μs       21.0±0.1μs     1.18  timeseries.Offsets.time_timeseries_day_apply
+      27.0±0.2μs      30.6±0.08μs     1.14  timeseries.Offsets.time_custom_bday_cal_decr
-     18.3±0.09μs       16.6±0.1μs     0.90  timeseries.AsOf.time_asof_single_early
-         816±3ns          734±2ns     0.90  period.PeriodProperties.time_minute
-     23.9±0.04μs      21.5±0.06μs     0.90  timeseries.AsOf.time_asof_single
-         294±5ms          258±6ms     0.88  timeseries.AsOfDataFrame.time_asof
-         298±7ms          255±6ms     0.86  timeseries.AsOfDataFrame.time_asof_nan
-         132±1μs        105±0.8μs     0.79  timeseries.Offsets.time_custom_bmonthbegin_decr_n
-       163±0.5μs        126±0.2μs     0.77  timeseries.Offsets.time_custom_bmonthbegin_incr_n
-         159±1μs        101±0.2μs     0.64  timeseries.Offsets.time_custom_bmonthend_decr_n
-         149±2μs      93.6±0.04μs     0.63  timeseries.Offsets.time_custom_bmonthend_incr_n
-       135±0.2μs       77.2±0.1μs     0.57  timeseries.Offsets.time_custom_bmonthend_incr
-      55.1±0.1μs      21.0±0.08μs     0.38  timeseries.SemiMonthOffset.time_end_decr_n
-     52.4±0.08μs      19.7±0.03μs     0.38  timeseries.SemiMonthOffset.time_begin_decr
-      53.7±0.1μs       20.2±0.1μs     0.38  timeseries.SemiMonthOffset.time_begin_decr_n
-     54.2±0.09μs      20.0±0.06μs     0.37  timeseries.SemiMonthOffset.time_end_decr
-     52.4±0.09μs       18.6±0.1μs     0.36  timeseries.SemiMonthOffset.time_begin_incr_n
-      53.6±0.4μs      18.2±0.07μs     0.34  timeseries.SemiMonthOffset.time_end_incr_n
-     48.4±0.06μs      15.4±0.05μs     0.32  timeseries.SemiMonthOffset.time_end_incr
-      48.5±0.2μs      15.4±0.03μs     0.32  timeseries.SemiMonthOffset.time_begin_incr
-     44.2±0.06μs      13.2±0.04μs     0.30  timeseries.SemiMonthOffset.time_end_apply
-      44.8±0.2μs      12.8±0.05μs     0.29  timeseries.SemiMonthOffset.time_begin_apply
-        26.9±2ms       7.22±0.2ms     0.27  timeseries.DatetimeIndex.time_dti_tz_factorize
-        26.6±2ms       6.65±0.2ms     0.25  timeseries.DatetimeIndex.time_dti_factorize
``` this looks reasonable. pls add a whatsnew note on improved perf of offsets, add all PRs that touch offsets.pyx to this issue. doesn't this supersede #18183 ? > this looks reasonable. pls add a whatsnew note on improved perf of offsets, add all PRs that touch offsets.pyx to this issue.

Will do. > doesn't this supersede #18183 ?

Yes.  Just closed that one. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=h1) Report
> Merging [#18218](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b36dab50a5dc7ca2d8de2b62a81014e095117771?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18218/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18218      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         163      163              
  Lines       50091    50091              
==========================================
- Hits        45788    45778      -10     
- Misses       4303     4313      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.36% <0%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.11% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.38% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=footer). Last update [b36dab5...21216a8](https://codecov.io/gh/pandas-dev/pandas/pull/18218?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Clipboard. thanks!