title: bokeh Hovertool tooltips showing date as number
body: <p>After making some transformations to a csv, I have a dataframe:</p>
  
  <pre><code>ENTRYDATE |  TRANSACTIONS
  2017/05/01     5
  2017/05/02     20
  ..
  2018/02/05     15
  </code></pre>
  
  <p>I converted it to datetime in pandas  and create a lineplot with tooltips with</p>
  
  <pre><code>import pandas as pd
  from bokeh.models import ColumnDataSource,DatetimeTickFormatter, NumeralTickFormatter, HoverTool
  from bokeh.plotting import figure
  from bokeh.io import curdoc
  
  df=pd.read_csv(r'C:\Users\file.csv')    
  df2=df[['ENTRYDATE']] 
  df2['ENTRYDATE']=pd.to_datetime(df2['ENTRYDATE'],infer_datetime_format=True)
  df2=(df2.groupby([df2['ENTRYDATE'].dt.date]).size().reset_index(name='Transactions'))
  #print(df2)
  #print(df2.info())
  
  source=ColumnDataSource(data=df2)
  
  #hovertool
  
  p=figure(plot_width=800,plot_height=500)
  
  hover=HoverTool(tooltips=[("Date","@ENTRYDATE"),("Transactions","@Transactions")],
  formatters={"ENTRYDATE":"datetime"},
  mode='vline')
  p.add_tools(hover)
  
  p.scatter(x="ENTRYDATE",y="Transactions",color='blue', source=source)
  p.xaxis.formatter=DatetimeTickFormatter()
  p.yaxis.formatter=NumeralTickFormatter(format="0")
  
  curdoc().add_root(p)
  </code></pre>
  
  <p>For some reason, the tooltip displaying the ENTRYDATE does not actually show the date (i.e. <code>2017-05-01</code>), rather it shows a some long number (i.e. <code>14962752000000</code>). Can someone tell me how to edit this so that the tooltips show the date in a date format?</p>