- Create new Google Apps Script
- Paste files here
- Set your calendar id and source calendar's ids and their titles in setting.gs
- Add Google Calendar API as a service from left side of the editor
- Share calendars with target calendar at Google Calendar
- Test and authorization
- Set trigger on GAS
Example
- syncToday() for 15 minutes
- syncThisMonth() for 1 hour
- syncThreeMonth() for 4 hours