Improve date and datetime picker functionality

[ahuang11]

Some bug fixes (start/end were not applied if datetime64) and makes the following work:

Closes #6143

import datetime as dt
import panel as pn
import numpy as np
import xarray as xr
import hvplot.xarray

pn.extension()

ds = xr.tutorial.open_dataset("air_temperature")
viz = ds["air"].hvplot(x="lon", y="lat")
pn.panel(viz, widgets={"time": pn.widgets.DatetimePicker}).show()

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (46e0639) 84.69% compared to head (7f01677) 84.70%.
Report is 8 commits behind head on main.

Files	Patch %	Lines
panel/widgets/input.py	87.50%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6152    +/-   ##
========================================
  Coverage   84.69%   84.70%            
========================================
  Files         296      296            
  Lines       44184    44294   +110     
========================================
+ Hits        37423    37520    +97     
- Misses       6761     6774    +13     

Flag	Coverage Δ
ui-tests	40.74% <48.68%> (+<0.01%)	⬆️
unitexamples-tests	72.60% <80.26%> (-0.02%)	⬇️

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[philippjfr]

Is this really needed or correct? What does bokeh convert the Date, Datetime values to?

[philippjfr]

Would prefer not to repeat this logic multiple times. Would pull this out as a utility maybe?

[ahuang11]

Actually I think there's an issue with the link between panel -> holoviews, and I'm not sure how to address:

Initially, the value is a np.datetime64

When a value is selected, it reverts back to datetime.datetime

Which I think is the root of a blank plot:

CC: @rsignell

[ahuang11]

Would it be acceptable to have a as_numpy_datetime64 param?

Screen.Recording.2024-01-09.at.1.46.32.PM.mov