title: How can I use datetime limits on X-Axis for Bokeh BoxAnnotation?
body: <p>I want to add BoxAnnotation to a plot that has a datetime x-axis. How can I set the left and right limits of of the BoxAnnotation to a datetime or date object. This is what I'm aiming for but it doesn't work.</p>
  
  <pre><code>from bokeh.sampledata.glucose import data
  from bokeh.models import BoxAnnotation
  from datetime import *
  
  # output_file("box_annotation.html", title="box_annotation.py example")
  
  TOOLS = "pan,wheel_zoom,box_zoom,reset,save"
  
  #reduce data size
  data = data.ix['2010-10-06':'2010-10-13']
  
  p = figure(x_axis_type="datetime", tools=TOOLS)
  
  p.line(data.index.to_series(), data['glucose'],
         line_color="gray", line_width=1, legend="glucose")
  
  left_box = BoxAnnotation(plot=p, right=date(2010,10,7), fill_alpha=0.1, fill_color='blue')
  mid_box = BoxAnnotation(plot=p, left=date(2010,10,8), right=date(2010,10,9), fill_alpha=0.1, fill_color='yellow')
  right_box = BoxAnnotation(plot=p, left=date(2010,10,10), fill_alpha=0.1, fill_color='blue')
  
  p.renderers.extend([left_box, mid_box, right_box])
  
  p.title = "Glucose Range"
  p.xgrid[0].grid_line_color=None
  p.ygrid[0].grid_line_alpha=0.5
  p.xaxis.axis_label = 'Time'
  p.yaxis.axis_label = 'Value'
  
  show(p)
  </code></pre>