Hi @ying zeng 

var gr = new GlideRecord('change_request');
gr.addQuery('first_field_date', gs.beginningOfToday()); 
gr.addQuery('hour_field', '08:00:00');
gr.query();
while(gr.next()) {
var gdt = new GlideDateTime(gr.getValue('first_field_date') +' '+gr.getValue('hour_field'));
gdt.addSeconds(-300);
var gdt1 = new GlideDateTime();
  if(gdt.equals(gdt1)) {
    gs.eventQueue("event_name", gr, "", "");
  }
}

Please first try to capture the first_field_date value and hour_field value in logs and check if it is giving the expected values or not. 

If my answer helps you to resolve your issue then hit correct or helpful.

Regards

Devender