There's time.ParseDuration which will happily accept negative durations, as per manual. Otherwise put, there's no need to negate a duration where you can get an exact duration in the first place.
E.g. when you need to substract an hour and a half, you can do that like so:
package main
import (
"fmt"
"time"
)
func main() {
now := time.Now()
fmt.Println("now:", now)
duration, _ := time.ParseDuration("-1.5h")
then := now.Add(duration)
fmt.Println("then:", then)
}