extracted a tweets since specific date

I try this code but 'since' is Unexpected parameter

# get tweets from the API
start_date = "2020-12-10T00:00:00Z"

tweets = tw.Cursor(api.search_tweets, q=hashtags, count=100, tweet_mode='extended', since= start_date).items(10)
# store the API responses in a list
tweets_copy = []
for tweet in tweets:
    tweets_copy.append(tweet)
    
print("Total Tweets fetched:", len(tweets_copy))