#18058: improve DatetimeIndex.date performance
- [x] closes #18058 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I ran asv with the tests from indexing.py, the output was "Benchmarks not significantly changed".

A quick test with %timeit:
```
In [1]: import pandas as pd

In [2]: rng = pd.date_range('2000-04-03', periods=200000, freq='2H')

In [3]: %timeit rng.date
555 ms ± 14.4 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [4]: %timeit rng.date_new
90.4 ms ± 5.79 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [5]: %timeit rng.normalize().to_pydatetime()
121 ms ± 3.08 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [6]: rng.date
Out[6]:
array([datetime.date(2000, 4, 3), datetime.date(2000, 4, 3),
       datetime.date(2000, 4, 3), ..., datetime.date(2045, 11, 19),
       datetime.date(2045, 11, 19), datetime.date(2045, 11, 19)], dtype=object)

In [7]: rng.date_new
Out[7]:
array([datetime.date(2000, 4, 3), datetime.date(2000, 4, 3),
       datetime.date(2000, 4, 3), ..., datetime.date(2045, 11, 19),
       datetime.date(2045, 11, 19), datetime.date(2045, 11, 19)], dtype=object)

In [8]: rng.normalize().to_pydatetime()
Out[8]:
array([datetime.datetime(2000, 4, 3, 0, 0),
       datetime.datetime(2000, 4, 3, 0, 0),
       datetime.datetime(2000, 4, 3, 0, 0), ...,
       datetime.datetime(2045, 11, 19, 0, 0),
       datetime.datetime(2045, 11, 19, 0, 0),
       datetime.datetime(2045, 11, 19, 0, 0)], dtype=object)
```

Please let me know if there are some other asv tests that need to be run. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=h1) Report
> Merging [#18163](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/103ea6fa33cbee3d9a193010016faa4cec7abf87?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18163/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18163      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.03%     
==========================================
  Files         163      163              
  Lines       49714    49714              
==========================================
- Hits        45415    45405      -10     
- Misses       4299     4309      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.13% <100%> (-0.01%)` | :arrow_down: |
| #single | `39.63% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `99.13% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.39% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18163?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/18163?src=pr&el=footer). Last update [103ea6f...3b7fa77](https://codecov.io/gh/pandas-dev/pandas/pull/18163?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can u add an asv for this? add a whatsnew note for 0.22, perf section Is the change in the timeseries.py file what you meant? Do you know how I can resolve the conflict? can u rebase again
sorry lots of merges happening yeah, i'm on it just finished it, do i need to push again? yes need to push again.  Hello @tmnhat2001! Thanks for updating the PR.







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

##### Comment last updated on November 22, 2017 at 04:46 Hours UTC can you rebase thanks @tmnhat2001  FYI @jbrockmendel IIRC you may have a PR to move some of this code to tslibs.conversion thanks @jreback
I'll open a pr for dti.time as well > IIRC you may have a PR to move some of this code to tslibs.conversion

@jreback #18389 is close, but shouldn't overlap with this.