bokeh server won't serve simple dateslider

Question

The following code should define a bokeh app that consists of a single dateslider. If I save it as html ( serve=False ) it works as expected but the bokeh server returns a blank page. Any clues?

s = datetime.strptime('2019-01-01 14:00:00', '%Y-%m-%d %H:%M:%S')
e = datetime.strptime('2019-01-01 15:00:00', '%Y-%m-%d %H:%M:%S')
v = datetime.strptime('2019-01-01 14:01:00', '%Y-%m-%d %H:%M:%S')
st = timedelta(seconds=7)
slider = DateSlider(start=s, step=2, end=e, value=e, format='%Y-%m-%d %H:%M')
mylayout = column(children=[slider])

if serve:
    curdoc().add_root(mylayout)

else:
    fname = os.path.join(...)
    output_file(fname)
    save(mylayout)

Further questions:

1. does the format string determine the lowest unit that changes when the slider is changed? (eg if I display seconds it will be seconds, if I display months it will be months)
2. what does step do in this case? It doesn't seem to make any difference what value I choose, the behaviour is the same, it just slides smoothely.
3. why is step required to be integral? I was expecting a timedelta value here.

Answer 1

All datetime values in Bokeh art typically in milliseconds, eg datetime axis values are milliseconds since epoch. Same in this case for intervals, eg here is an increment of 5 minutes:

from bokeh.models import DateSlider
from bokeh.io import curdoc
from bokeh.layouts import column
from datetime import datetime, timedelta

s = datetime.strptime('2019-01-01 14:00:00', '%Y-%m-%d %H:%M:%S')
e = datetime.strptime('2019-01-01 15:00:00', '%Y-%m-%d %H:%M:%S')
v = datetime.strptime('2019-01-01 14:01:00', '%Y-%m-%d %H:%M:%S')
st = timedelta(seconds=7)
slider = DateSlider(start=s, step=1000*60*5, end=e, value=e, format='%Y-%m-%d %H:%M')
mylayout = column(children=[slider])

curdoc().add_root(mylayout)

Having the step accept timedelta seems like a reasonable thing. Bokeh is huge project and many nice conveniences are not implemented simply because no one has ever pointed them out before. This would be a good issue to make aon GitHub, especially if you are interested in helping contribute.