ConversionError: Failed to convert value(s) to axis units: '2015-01-01'

Question

I am trying to convert values to axis units. I checked codes with similar problems but none addressed this specific challenge. As can be seen in the image below, expected plot (A) was supposed to show month (Jan, Feb etc.) on the x-axis, but it was showing dates (2015-01 etc) in plot (B).

Below is the source code, kindly assist. Thanks.

plt.rcParams["font.size"] = 18

plt.figure(figsize=(20,5))
plt.plot(df.air_temperature,label="Air temperature at Frankfurt Int. Airport in 2015")
plt.xlim(("2015-01-01","2015-12-31"))
plt.xticks(["2015-{:02d}-15".format(x) for x in range(1,13,1)],["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"])
plt.legend()
plt.ylabel("Temperature (°C)")
plt.show()