Is it possible to add just a time as a custom data field for an event?
You could create custom text fields, then add these 3 lines of javascript somewhere to your site where it will get loaded on every CiviCRM page:

CRM.$(document).on('crmLoad', function(e) {
  CRM.$('input[data-crm-custom="Event_Time:Start_Time"], input[data-crm-custom="Event_Time:Start_Time"]', e.target).timeEntry();
});


This assumes you've named the custom group "Event Time" and the fields "Start Time" and "End Time". Adjust the code as needed to match the actual names of your fields (while creating/editing an event, right-click on a field and "Inspect" it - in your console look for the data-crm-custom attribute).

The .timeEntry() widget will store the data in human readable format (e.g. 12:53PM). If you prefer it to be stored in a searchable format (so you can sort your events by time, etc) use .crmDatepicker({date: false}) instead of .timeEntry().

Also be careful you don't load this snippet on every page of your website as it will cause an error on non-civi pages due to the lack of CRM variable. hook_civicrm_coreResourceList is a good choice for achieving this.