@muendlein Bokeh already supports multiple levels of datetime tick format context:

https://docs.bokeh.org/en/latest/docs/user_guide/styling/plots.html#datetime-tick-context

Are you proposing that this existing feature be used by default? I'm not sure there is anything that will satisfy everyone here, which makes me slightly inclined to preserve the status quo and simply make customization features as accessible and known a possible.

@bryevdv Thank you for pointing out that multiple levels are already available!
Of course I am fully aware of that not everyone can be satisfied but I think the default settings should be designed to satisfy most which I don't really see right now. In particular when there are established implementations that do it differently (better).

My proposal is two-fold:

• Utilize the secondary level to provide more information. I think this should be a no-brainer because it just simply provides additional information. Currently when zoomed in and you will see only hours and minutes without any date context which is not helpful. Note: It's also worth pointing out that this is pretty common in order libraries/languages (i.e. plotly, Matlab).
• Do not used mixed formats for tick labels on the same level, i.e.
  
  This is an absolute nightmare in terms of readability and I cannot see why anyone would really want that.

Both of my points are essentially covered by the plotly library (example: https://plotly.com/python/time-series/#time-series-with-range-slider).
Example screenshots of different zoom levels from the plotly example:

@muendlein Did you want to submit a PR for consideration? Speaking plainly I think this will move much faster, even it if requires some iteration and back and forth, if someone champions a concrete proposal to start from.