text: Title: get_portfolio_history TypeError:: Cannot convert Float64Index to dtype datetime64[ns]
  Body: I've used this for months to check my account's current profit/loss % for the day:
  
  ```
    # Connect to the Alpaca API
    alpaca = tradeapi.REST(API_KEY, API_SECRET, APCA_API_BASE_URL, 'v2')
  
    # Define our destination time zone 
    tz_dest = pytz.timezone('America/New_York') 
  
    # Get the current date
    today = datetime.datetime.date(datetime.datetime.now(tz_dest))
    
    # Get today's account history
    history = alpaca.get_portfolio_history(date_start=today, timeframe = '1D', extended_hours = False).df
  
    # Format profit/loss as a string and percent        
    profit_pct = str("{:.2%}".format(history.profit_loss_pct[0]))    
  ```
  
  As of today though, I'm getting this error when I try to put any date in the "date_start" command of the get_portfolio_history function. I haven't been able to find a date format it'll take yet. I keep hitting this error:
  
  **TypeError: Cannot convert Float64Index to dtype datetime64[ns]; integer values are required for conversion**
  
  It seems that the error from these two earlier issues has re-surfaced. Help? 
  
  https://github.com/alpacahq/alpaca-trade-api-python/issues/62#issue-422913769
  https://github.com/alpacahq/alpaca-trade-api-python/issues/53#issue-405386294