You have been able to divide a time.Duration by an integer since before go 1.0. As you can see in the stdlib, time.Duration is just an int64 (https://golang.org/pkg/time/#Duration). You do have to cast the integer to a time.Duration sometimes.

Here's a playground showing cases where it works and cases where it require a cast: https://play.golang.org/p/6Pbqrz8ZZ3t