Adding 30 minutes to a Google calendar entry

Hi

I have events un Google Calendar that have a Event Time start and end but then it has a meet time, which you put how long before start time you want to meet.

When I have events in from the App, it pulls the start time including the meet time.

Below is my start time. How can I get that to add 30 minutes to the time, so I have a meet time, start time and a finish time please ?

Ive tried various but it keeps saying I cant add a string to an INT

Thanks

Martyn

- platform: template
  sensors:
    jaguars_next_game_kickoff:
      value_template: "{{ as_timestamp(strptime(state_attr('calendar.jaguar_games', 'start_time'), '%Y-%m-%d %H:%M:%S')) | timestamp_custom('%H:%M') }}"
      friendly_name: "Kick Off"

anyone able to help please ?