I am writing a trigger where I am bringing in a Date/Field field from case. It has a letter in it 2023-02-10T16:15:35.000+0000 and when I try to subtract it with "DateTime.now()" I get an error "Date/time expressions must use Integer or Double or Decimal ".

This is my code 
trigger CaseTrigger on Case (before update) {
     for(Case cases: Trigger.new){ 
         if(cases.stopped_time__c != null){
             cases.Total_amount_of_time_in_Hours__c= (DateTime.now()-cases.stopped_time__c );
         } 
    }
}