How to convert -99 value in datetime column to a date of my choosing? [duplicate]

I have some data:

ID	Date
1	2021-05-01
2	2021-05-01
3	2021-05-01
4	-99
5	-99
6	2017-11-21
7	-99
8	2019-02-04
9	-99
10	-99

Where I have -99, I want to change it to 2021-05-20. I have tried if statement but it won't work, there are still -99s. I have made sure it is a datetime column although thinking about that -99 isn't a datetime value. Maybe that is where I'm going wrong?

df['Date'] = pd.to_datetime(df['Date'], format='%Y-%m-%d')

if (df['Date'] is '-99'):
    df['Date'] = pd.Series(['2021-05-20' for x in range(len(df.index))])

I'm thinking the length part is wrong too, since there aren't as many -99 as there are data rows. Thanks in advance for the help!