Test for #16726. unittest that ensures datetime is understood
 - [x] closes #16726
 - [x] test added

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16744?src=pr&el=h1) Report
> Merging [#16744](https://codecov.io/gh/pandas-dev/pandas/pull/16744?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8a98f5ed541c87a9bf101c9331bd6cfa8f007cc9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16744/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16744?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16744   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         161      161           
  Lines       49282    49282           
=======================================
  Hits        44816    44816           
  Misses       4466     4466
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.7% <ø> (ø)` | :arrow_up: |
| #single | `40.17% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16744?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/16744?src=pr&el=footer). Last update [8a98f5e...2fb8782](https://codecov.io/gh/pandas-dev/pandas/pull/16744?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/16744?src=pr&el=h1) Report
> Merging [#16744](https://codecov.io/gh/pandas-dev/pandas/pull/16744?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8a98f5ed541c87a9bf101c9331bd6cfa8f007cc9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16744/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16744?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16744   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         161      161           
  Lines       49282    49282           
=======================================
  Hits        44816    44816           
  Misses       4466     4466
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.7% <ø> (ø)` | :arrow_up: |
| #single | `40.17% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16744?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/16744?src=pr&el=footer). Last update [8a98f5e...36960cd](https://codecov.io/gh/pandas-dev/pandas/pull/16744?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!