Apex class:
public class Deactiveusercls {
@future    
public static void processAsync() {
    
     DateTime dt = DateTime.now();
        String dateString = dt.format().replace('/',' ');
       System.debug(dateString);
      String[] strDTDivided = dateString.split(' ');
        system.debug('strDTDivided'+strDTDivided);
       string month = strDTDivided.get(0);
       system.debug('month'+month);
      string day = strDTDivided.get(1).replace(',', '');
       system.debug('day'+day);
      string year = strDTDivided.get(2);
      system.debug('year'+year);

      string hour = strDTDivided.get(3).split(':').get(0);
      string minute = strDTDivided.get(3).split(':').get(1);
      string second = '00';
      String ampm = strDTDivided.get(4);
    /*  if (strDTDivided.get(4) == 'PM')
     {
     hour = String.ValueOf(Integer.ValueOf(hour) + 12);
     }*/
   // string stringDate = year + '-' + month + '-' + day + ' ' + hour + ':' + minute + ':' + second;
   string stringDate = year + '-' + month + '-' + day + ' ' + hour + ':' + minute + ' ' +ampm; 
    system.debug('stringDate'+stringDate);
    
    map<string, Deactiveuser__c> ObjMap = new map<string, Deactiveuser__c>();
    LIst<Deactiveuser__c> activeLst =[select id,UserName__c,Name__c,TerminateDate__c from Deactiveuser__c where TerminateDate__c =:stringDate];
    system.debug('activeLst'+activeLst);
    for(Deactiveuser__c obj: activeLst)
    {
        if (obj.UserName__c != Null)
        {
            ObjMap.put(obj.UserName__c, obj);
        }
    }
    System.debug('ObjMap'+ObjMap);
    List<User> users = [SELECT isActive,email,Username,Alias FROM User WHERE Alias IN :ObjMap.KeySet()  and  isActive = true];
    system.debug('users'+users);
    List<User> userUpdateList = new List<User>();
   // String dateFormat = 'yyyy-MM-dd\'T\'HH:mm:ss\'Z\''.replace('\'T\'',' ').replace('\'Z\'','');
    for(User u: users)
    {
        u.isActive = false;
        userUpdateList.add(u);
   }
    if(userUpdateList.size() > 0)
    {
        update userUpdateList;
        system.debug('userUpdateList'+userUpdateList);
    }  
}   
}

Test Class:

@isTest
public class DeactiveuserclsTest {
 static testMethod void deactive() {
   
     
     
      Profile pf= [Select Id from profile where Name='System Administrator']; 
     User u = new User(
     ProfileId = pf.Id,
     LastName = 'last',
     Email = 'puser000@amamama.com',
     Username = 'puser000@amamama.com' + System.currentTimeMillis(),
     CompanyName = 'TEST',
     Title = 'title',
     Alias = 'test',
     TimeZoneSidKey = 'America/Los_Angeles',
     EmailEncodingKey = 'UTF-8',
     LanguageLocaleKey = 'en_US',
     LocaleSidKey = 'en_US'
  
     );
     insert u;
     
     Deactiveuser__c d=new Deactiveuser__c();
     d.Name__c ='testUser';
     d.UserName__c = 'test';
     d.TerminateDate__c = '2022-4-25 6 PM';
     insert d;
    
     u.IsActive = false;
    // update u;
    
    Test.startTest();
     Deactiveusercls.processAsync();
      Test.stopTest(); 
     
//   User[] users = [SELECT Id from User where username='mruiz@awcomputing.com'];
        System.assertEquals(false, u.IsActive);  
     }
}