Pymysql can only concatenateエラー

前提・実現したいこと

target_timeをwhere句で使いたいのですが、エラーが出ています。
おそらくtarget_timeの使い方が悪いと思うのですが、行き詰ってしまっています。

発生している問題・エラーメッセージ

can only concatenate str (not "datetime.datetime") to str

該当のソースコード

Python 3.7.6

import json
import boto3
import os
from datetime import datetime
import pytz
import pandas as pd
import io
import pymysql
import csv
import numpy as np
import time
import shutil
from datetime import timedelta

dtformat = "%Y/%m/%d-%H:%M:%S.%f"
apptz    = "Asia/Tokyo"
dbtz = "UTC"

RDS_HOST = "HOST"
RDS_USERNAME = "USER"
RDS_PASS = "PASS"
RDS_DBNAME = "DBNAME"

conn = pymysql.connect(RDS_HOST, user=RDS_USERNAME, passwd=RDS_PASS, db=RDS_DBNAME, connect_timeout=5)

ls_code = {"AAA","BBB"}
ls_name = {"CCC","DDD"}

def main():
    try:
        for line_code,line_name in zip(ls_line_code, li_line_name):
            with conn.cursor() as cursor:
                now_time = datetime.now()
                target_time = now_time - timedelta(hours=96)

                sql_switch_data = "SELECT tst.time_start, tst.time_end, tst.required_time, ilr.p_item_code, ilr.a_item_code, tst.w_code " +\
                        "FROM TST tst " +\
                        "INNER JOIN ILR ilr ON tst.id = ilr.id " +\
                        "INNER JOIN CTP ctp ON ilr.code = ctp.code " +\
                        "WHERE time_start >=  '" + target_time + "' AND ctp.code = '" + line_code + "'"