How to get date field object value?
Try
  
  $node->get('start_time')->getValue();


This actually gives you an array, if you inspect/dump the array, then you'll see that you can get the value like this.

$node-&gt;get('start_time')-&gt;getValue()[0]['value'];

Alternatively, you can simply use $node-&gt;start_time-&gt;value;

Note: I am using the default Date field from Core and assuming you're using a Node. Hence the $node, and also worth noting, usually your date machine name will be field_something. Which makes me believe that OPer is using some contrib or custom module.

If you're using some contrib module that allows you to have start and end times on your date field, probably will require a diff solution.