An update to Google Calendar is rolling out soon that will allow you to add Out-of-Office time to your calendar. Invites received during this time will be automatically declined with the message of your choice (or no message at all, but isn’t that rude?)
Also included with this enhancement is a more flexible ‘working hours’ settings. Currently you can only set one repeating block of working time. The enhanced calendar will allow for different set hours on different days, which works well for those with flex-time schedules.
This update will first appear for the paying (G-Suite) customers in the next few days, and roll out to the rest of us shortly after.