Seaborn regplot using datetime64 as the x axis

I have a dataframe looks like this:

date         score  
2017-06-04    90
2017-06-03    80
2017-06-02    70

When I tried this:

sns.regplot(x=date, y=score, data=df)

I got an error:

TypeError: reduction operation 'mean' not allowed for this dtype

The dtype for date is datetime64[ns], and int64 for the score column.

How can I covert the date column so that regplot will work?