BUG: Groupby Is Changing Date Format
- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```
df = pd.read_csv("Uk-Orders.csv",parse_dates=['WO Created'])
df=df.rename(columns={"WO Created":"WO-Created","Sale Price":"Sale-Price"})
df['Sale-Price']=df['Sale-Price'].str.replace(',', '').astype('float64')
df['WO-Created'] =  pd.to_datetime(df['WO-Created']).dt.normalize()
df=df.groupby('WO-Created',sort=False)['Sale-Price'].sum().reset_index()
```

#### Problem description

I have a dataset that has two columns. WO created as starting on 04-01-2016 and are populated until 06-29-2020. 
After running df=df.groupby('WO-Created',sort=False)['Sale-Price'].sum().reset_index(), the last row is giving me dates like 2020-12-06, which is incorrect, because there is no such date range in my report (no December in my dataset). I suppose it has something to do with date formatting, but I am not sure how to fix it.

#### Expected Output

I would expect dates to be summarized correctly.

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here leaving a blank line after the details tag]

</details>
