//a controller class to get incidents for display on VF page and then
//anonymise the incidents and related contacts
public class incidentAnonController {

public Incident__c firstDate{get; set;}
public Incident__c lastDate{get; set;}

public incidentAnonController(ApexPages.StandardController controller){
    Integer dy = System.today().day();
    Integer mo = System.today().month();
    Integer yr = System.today().year();
    firstDate = new Incident__c(Date_Of_Incident__c = Date.newInstance(yr, mo, dy).addMonths(-6));
    lastDate = new Incident__c(Date_Of_Incident__c = Date.newInstance(yr, mo, dy).addMonths(-12));
}

//method to anonymise relevant records
public void anonymise() {
    //list of incidents older than 6 months. These will be anonymised.
    List<Incident__c> results = [SELECT Id, Incident_name__c, Date_of_incident__c, Name, Garda_Name__c
     FROM Incident__c WHERE Date_of_Incident__c <= :firstDate.Date_Of_Incident__c AND
     Date_of_Incident__c >= :lastDate.Date_Of_Incident__c];


    //anonymise incidents that are older than 6 months
    for (Incident__c inc : results) {
        inc.Incident_name__c = inc.Name;
        inc.Garda_Name__c = '';
        inc.Pulse_ID__c = '';
    }
    update results;

@isTest
public class incidentAnonTest {

public static testMethod void incidentAnon() {
    Integer dy = System.today().day();
    Integer mo = System.today().month();
    Integer yr = System.today().year();

    //create an incident
    Incident__c inc1 = new Incident__c();
    inc1.Date_Of_Incident__c = Date.newInstance(yr, mo, dy).addMonths(-7);
    inc1.Incident_name__c = 'Incident 1';
    inc1.Garda_Name__c = 'Test Garda';
    insert inc1;

    PageReference pageRef = Page.AnonymiseIncidents;
    Test.setCurrentPage(pageRef);

    ApexPages.StandardController sc = new ApexPages.StandardController(inc1);
    incidentAnonController testInc = new incidentAnonController(sc);

    inc1 = [SELECT Id, Garda_Name__c, Date_of_Incident__c, FROM Incident__c WHERE Id = :inc1.Id];

    testInc.anonymise();

    System.debug(inc1.Date_Of_Incident__c);
    System.debug(inc1.Garda_Name__c);

    System.assertEquals(TRUE, String.isBlank(inc1.Garda_Name__c));

}

<apex:page standardController="Incident__c" extensions="incidentAnonController" docType="html-5.0">
<apex:form >

    <apex:pageBlock title="Notes on Usage">
        <apex:pageBlockSection >
            //some content
        </apex:pageBlockSection>
    </apex:pageBlock>

    <apex:pageBlock title="Date Ranges for Anonymisation">
        <apex:pageBlockSection >
            <apex:inputField value="{!firstDate.Date_Of_Incident__c}" label="From (Default 6 Months ago)"/>
            <apex:inputField value="{!lastDate.Date_Of_Incident__c}" label="To (Default 1 year ago)"/>
        </apex:pageBlockSection>
    </apex:pageBlock>

    <apex:pageBlock title="List of Incidents">
            <apex:pageBlockTable value="{!incidents}" var="inc">
                <apex:column headerValue="Incident Number">
                    <apex:outputLink value="/{!inc.id}">{!inc.Name}</apex:outputLink>
                </apex:column>
                <apex:column value="{!inc.Incident_Name__c}"/>
                <apex:column value="{!inc.Date_Of_Incident__c}"/>
            </apex:pageBlockTable>

            <apex:pageBlockButtons >
                <apex:commandButton action="{!anonymise}" value="Anonymise"/>
                <apex:commandButton onclick="location.reload();" value="Update List"/>
            </apex:pageBlockButtons>

    </apex:pageBlock>
</apex:form>