Solved my problem, though it is quite convoluted
I got the timestamp from the obspy UTC function, then converted it from timestamp to the python dates format, then converted that dat format to a number. This worked. What I wrote is below.
Any easier way to do this I’d love to hear.
import matplotlib.dates as dates
from obspy import UTCDateTime
from obspy.taup import TauPyModel
import datetime
ev_time = "2020-07-22 06:12:44"
ev_depth = 28
ev_dist = 69.6
t = UTCDateTime(ev_time)
model = TauPyModel(model = 'iasp91')
arrivals = model.get_travel_times(source_depth_in_km = ev_depth, distance_ in_degree = ev_dist)
arr_time = t + arrivals[0].time
arr_time2 = datetime.datetime.utcfromtimestamp(arr_time)
arr_time3 = dates.date2num(arr_time2)
# Then later during the plotting part of the script
plt.axvline(x=arr_time3)