Cannot write to Excel worksheet with valid date format with Pandas

The background:

1, used Python to execute the SQL and saved the output into DataFrame format using Pandas.

2, appended the output to my existing excel file as one new worksheet.

Below is my code:

from pandas import ExcelWriter

sql_20 = ''''''

db = cx_Oracle.connect('*****', '*******', '**********')
conn = db.cursor()

conn.execute(sql_20)
df = pandas.read_sql_query(sql_20,db)
print(df)

with ExcelWriter('GUCS6J-Job Data.xlsx', mode='a') as writer:
    df.to_excel(writer, sheet_name=str(20))
writer.save()

Database: Oracle

Column A data type: date

The SQL result from Oracle:

The DataFrame print from Python:

So far so good

But what I got in the excel worksheet:

I am curious why I got the '##################' and the value of this cell is -693594 in the excel.

I hope I can get the same result as the DataFrame print in the excel file.