Datetimes on plot are always treated as local time and shifted to UTC

Description

When displaying datetime.datetime values along the x-axis using x_axis_type = "datetime", the values are shifted if the system’s timezone is not UTC.

For example, if you live in UTC-05:00, then all the dates are shifted ahead 5 hours (7am appears at noon).

This happens regardless of the tzinfo on the datetimes, including for naive datetimes where tzinfo is None.

Sample Code

from datetime import datetime
from bokeh.plotting import figure, show
from bokeh.models.sources import ColumnDataSource
from bokeh.models.tools import CrosshairTool, HoverTool

hours = range(24)
datetimes = [datetime.now().replace(hour=h, minute=0, second=0, tzinfo=None) for h in hours]

example = figure(
	x_axis_label = 'Date/Time',
	x_axis_type = 'datetime',
	y_axis_label = 'Expected Hour',
)
example.circle(
	x = 'x',
	y = 'y',
	fill_color = 'blue',
	size = 6,
	source = ColumnDataSource({
		'x': datetimes,
		'y': hours,
		'time_fmt': [dt.strftime("%H:%M") for dt in datetimes],
	}),
)
example.add_tools(CrosshairTool())
example.add_tools(HoverTool(
	tooltips = [
		("index", "$index"),
		("expected time", "@time_fmt"),
	],
))

show(example)

Expected Behaviour

I would have expected that naive datetimes are displayed as is.

Actual Behaviour

System, Library, Browser, Version information

I am using Python 3.5.2 (64-bit) with bokeh 0.12.3 and Firefox 50.0 as well as Chrome 54.0.2840.100 (64-bit) on Ubuntu MATE 16.04.1 LTS and on CentOS release 6.8.

bokeh info (Ubuntu):

Python version      :  3.5.2 (default, Sep 10 2016, 08:21:44) 
IPython version     :  5.1.0
Bokeh version       :  0.12.3
BokehJS static path :  /usr/local/lib/python3.5/dist-packages/bokeh/server/static

bokeh info (CentOS):

Python version      :  3.5.2 (default, Nov 24 2016, 13:03:33) 
IPython version     :  Not installed
Bokeh version       :  0.12.3
BokehJS static path :  /usr/local/lib/python3.5/site-packages/bokeh/server/static

Other notes

You can work around this issue by setting your system timezone to UTC. This is not ideal though.

[bryevdv]

I just pushed this change to the linked PR. With this change, the original code now yields:

@mwerezak @gismo2004 any chance you can test the PR out by cloning and building bokeh locally?

[mwerezak]

Looks like gismo2004 beat me to it.