TST: lock down timeseries now tests, xref #18666
See the comment attached to `test_to_datetime_today`.  This test is not deterministic, will fail to detect the change introduced by #18666 1 hour out of each day.

- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=h1) Report
> Merging [#18709](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/34a8d36e45c1623d2e61671561cdbd5de36adac8?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18709/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18709      +/-   ##
==========================================
- Coverage    91.6%   91.56%   -0.05%     
==========================================
  Files         153      153              
  Lines       51273    51273              
==========================================
- Hits        46970    46947      -23     
- Misses       4303     4326      +23
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.42% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.68% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18709/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18709/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `64.78% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18709/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.82% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18709/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18709?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/18709?src=pr&el=footer). Last update [34a8d36...75fadb8](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=h1) Report
> Merging [#18709](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4091f64d6e0dca3177f905b71f4d067bef8acd91?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18709/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18709      +/-   ##
==========================================
- Coverage   91.61%   91.59%   -0.02%     
==========================================
  Files         153      153              
  Lines       51361    51361              
==========================================
- Hits        47053    47046       -7     
- Misses       4308     4315       +7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.46% <ø> (ø)` | :arrow_up: |
| #single | `40.73% <ø> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18709/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18709/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18709/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.53% <0%> (+0.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18709?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/18709?src=pr&el=footer). Last update [4091f64...af66e44](https://codecov.io/gh/pandas-dev/pandas/pull/18709?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 failing on windows. Weird since only comment/whitespace change since last run.  Error is in pandas.util.testing, may just have to skip on windows. thanks!