halcyon days (noun) 

In Greek mythology, the halcyon (HAL-see-un) bird was said to calm the wind and the waves. The phrase “halcyon days” refers to a tranquil period of happiness, success, and prosperity, especially in the past. As I used it today: “My early experiences [with Uber] made me a big fan and a loyal customer…. Things have changed since those halcyon days.”