Matplotlib - Horizontal Bar Chart Timeline With Dates - Xticks not showing date

You can plot each bar as line, choosing the width of the line (lw) you prefer:

# Set the color of the grid lines
mpl.rcParams['grid.color'] = "w"

fig, ax = plt.subplots(1, 1)
# Plot eac item as a line
for i, (b, e, l) in enumerate(zip(beg_sort, end_sort, evt_sort)):
    ax.plot_date([b, e], [i + 1] * 2, ls='-', marker=None, lw=10)  # 10 for the line width

# Set ticks and labels on y axis
ax.set_yticks(range(1, len(evt_sort) + 1))
ax.set_yticklabels(evt_sort)

# Set color and transparency of the grid
ax.patch.set_facecolor('gray')
ax.patch.set_alpha(0.3)
# activate grid
ax.grid(True)

Moreover, you can play with the background grid, customizing it according to your needs.

Hacked something together that works, posting for curiosity, however go with PieCot's answer above:

import matplotlib.pyplot as plt
import numpy as np
from datetime import date
from datetime import datetime
import matplotlib.dates as mdates
#https://stackoverflow.com/questions/58387731/plotting-month-year-as-x-ticks-in-matplotlib
fig, ax = plt.subplots(ncols=2, nrows=1, figsize=(15, 4.18))

#fig, ax = plt.figure( figsize=(15, 4.18))

event = np.array(['Groupone','Grouptwo','Group3','Group4','Group5','Group6'])
begin = np.array([datetime(year=2019,month=12,day=1),datetime(year=2020,month=2,day=1),datetime(year=2020,month=5,day=1),datetime(year=2020,month=11,day=1),datetime(year=2019,month=12,day=1),datetime(year=2020,month=5,day=1)])
end = np.array([datetime(year=2019,month=12,day=30),datetime(year=2021,month=2,day=1),datetime(year=2021,month=2,day=1),datetime(year=2021,month=2,day=1),datetime(year=2021,month=2,day=1),datetime(year=2020,month=7,day=3)])

beg_sort = np.sort(begin)
end_sort = end[np.argsort(begin)]
evt_sort = event[np.argsort(begin)]
#start_m     = click.prompt('Start month', type=int)
#start_y     = click.prompt('Start year', type=int)
#end_m       = click.prompt('End month', type=int)
#end_y       = click.prompt('End year', type=int)
start_m     = 12
start_y     = 2019
end_m       = 3
end_y       = 2021
months = mdates.MonthLocator()          # Add tick every month
#days = mdates.DayLocator(range(1,32,5)) # Add tick every 5th day in a month
#monthFmt = mdates.DateFormatter('%b')   # Use abbreviated month name
ax[1].xaxis.set_major_locator(months)
#ax[1].xaxis.set_major_formatter(mdates.DateFormatter('%m-%Y')) 
ax[1].xaxis.set_major_formatter(mdates.DateFormatter('%b-%Y')) 

ax[1].xaxis.set_tick_params(rotation=90)
#ax.xaxis.set_tick_params(rotation=30)
#ax[1].xaxis.set_major_formatter(monthFmt)
#ax[0].xaxis.set_minor_locator(days)
start       = date(year=start_y,month=start_m,day=1)
print(start)
end         = date(year=end_y,month=end_m,day=1)
print(end)
Nticks      = 6
delta       = (end-start)/Nticks
tick_dates  = [start + i*delta for i in range(Nticks)]
x_ticks     = ['{}/{}'.format(d.month,d.year) for d in tick_dates]
print(x_ticks)
plt.barh(range(len(beg_sort)), end_sort-beg_sort, left=beg_sort, align='center')
plt.yticks(range(len(beg_sort)), evt_sort)

plt.yticks(range(len(beg_sort)), evt_sort)
#plt.xticks = x_ticks
#plt.set_xticks(x_ticks)

#plt.show()
fig.delaxes(ax[0])

plt.savefig('gwern.pdf',bbox_inches='tight')