I have to modify a xml file and i am using xml e tree

I have got a very big xml document and i have to modify the value of 2 tags there.

                </Hours>
                <IsBusyHourSet>true</IsBusyHourSet>
                <BusyHour>-1</BusyHour>
                <DayOfWeek>
                  <Day>0</Day>
                </DayOfWeek>
              </DayDetails>
            </DayDetailsList>
            <DayHourType>Specified</DayHourType>
            <IsUsingAllDaysAndHours>true</IsUsingAllDaysAndHours>
          </DayHourSettings>
        </Value>
      </AnalysisParameter>
      <AnalysisParameter Id="f7c95" IsPrivate="false" IsReadOnly="false" IsDeletable="true" PromptOnTemplate="false" IsPartitionable="false">
        <Name>Time</Name>
        <Description>hourly</Description>
        <CsvInputReferenceName />
        <Value Type="Resources.Utilities.DefaultTimePeriod, Wpf.Resources">
          <DefaultTimePeriod>
            <Type>SpecificDates</Type>
            <SpecificDateStart>2022-09-15T00:00:00</SpecificDateStart>
            <SpecificDateEnd>2022-09-15T23:59:59.999</SpecificDateEnd>
            <RelativeTimeValue>1</RelativeTimeValue>
            <RelativeTimeType>Days</RelativeTimeType>
            <RelativeType>TheLast</RelativeType>
          </DefaultTimePeriod>
        </Value>
      </AnalysisParameter>
    </AnalysisParameters>
  </ParameterStoreForSerialization>

I have to change the value of "SpecificDateStart" and "SpecificDateEnd" (tagname is not unique) which is existing multiple times in documents but i have to change at a specific place only which i have attached

I am using this code

from xml.etree import ElementTree as ET
tree = ET.parse(Document.xml)
tree.find('.//SpecificDateStart').text = '1/1/2011'

This is working but it is changing the first instance of "SpecificDateStart" as the tag name is not unique I have to change it at the 3rd or 4th instance for which iam using another code.

tree.find('/DefaultTimePeriod/SpecificDateStart').text = '1/1/2011'

which is showing me error

'NoneType' object has no attribute 'text'