An engineer SHALL have hack days.

Google does it. Atlassian does it. Everyone does it, and so should you.

The benefits have been written about time and time again.

Plan well, and have fun, and ensure that the hack lives on: as much fun as hack days are, developers get gratification from seeing their time investment go beyond the awards ceremony.

Also: Keynes said that by this time none of us should really be working, and since we're only getting paid £100K per annum I think we can all agree that an extra day a week of playtime won't hurt. Yes, CEO - we know we're making you rich.