从日期到字符串的Pyspark类型转换问题

我正在使用pyspark 2.1。下面是我的数据框架内容

expecteddays,date
139,30.JUl.2017
134,01.NOV.2018
我的输出应该是这样的
138,30.JUL.2017,<30/SEP/2018,4/FEB/2019>
最后一列的计算由我的模块dateRangeBetween和get_date负责。
以下是我的代码
from datetime import datetime 
from datetime import timedelta
import pandas as pd
from datetime import timedelta
from pyspark.sql import SparkSession
from pyspark import SparkContext
from pyspark.sql.functions import concat,explode
from datetime import datetime
from pyspark.sql.functions import udf
from pyspark.sql.types import  StringType
from datetime import timedelta
import pandas as pd
from pyspark.sql.types import ArrayType, StructType, StructField, IntegerType
from pyspark.sql import types   maintenance_final_join=spark.read.csv('/user/NaveenSri/adh_dev_engg/test.csv',header=True)
def get_date(dateFormat="%d-%m-%Y", addDays=0 ,timeNow=0 ): 
    #print('inside get date',timesNow)
if (addDays!=0):
        anotherTime = timeNow + timedelta(days=addDays)
    else:
        anotherTime = timeNow
    return anotherTime.strftime(dateFormat)
def dateRangebetween(expectedDate , estimatedDays):
output_format = '%d-%m-%Y'
dateRangeList =[]
#print('inside Date range',expectedDate)
rangeEnddate= datetime.strptime(get_date(output_format, 730,expectedDate), '%d-%m-%Y').date()
#print('rangeEnddate---',rangeEnddate)
calculatedDate = datetime.strptime(get_date(output_format,estimatedDays ,expectedDate), '%d-%m-%Y').date()
#print('calculatedDate----',calculatedDate)
while(calculatedDate<=rangeEnddate):    
   # print(calculatedDate)
#print (estimatedDays)  
    dateRangeList.append(calculatedDate)
    calculatedDate = datetime.strptime(get_date(output_format,estimatedDays ,calculatedDate), '%d-%m-%Y').date()
#print('-----', datetime.strptime(get_date(output_format,estimatedDays ,calculatedDate), '%d-%m-%Y').date())  
return dateRangeList
dateRange = udf(dateRangebetween, types.ArrayType(types.StringType()))
addDays=182
result = maintenance_final_join.withColumn('Part_Dates',dateRange(maintenance_final_join.Expected,maintenance_final_join.estimateddays)).show()
执行后，我得到了这个错误。
TypeError: coercing to Unicode: need string or buffer, datetime.timedelta found