//allcandidates is a list of records

for(m=0;m<allcandidates.size()-1; m++) {

 System.debug(' m' + allcandidates[m]);  //2019-01-02 04:18:00
 System.debug('m+1' + allcandidates[m+1]); // 2013-02-19 04:19:00

 if(allcandidates[m].Checkin_Date__c <= allcandidates[m+1].Checkin_Date__c)


  {
     System.debug('Condition is true')

   }

List<MyObject__c> records = [
    SELECT ...
    FROM MyObject__c
    WHERE ...
    ORDER BY Checkin_Date__c DESC
];