potential solution to https://exercism.io/my/solutions/e172d0beff604ee5ac1345e55ab92eb4

clock.go

package clock 

import (
    "fmt"
)

type clock struct {
  hour int
  minute int
} 

func New(hour, minute int ) clock {
	hour, minute = build(hour, minute)
  return clock{hour:hour, minute:minute}
}

func (c clock) String() string {
  return fmt.Sprintf("%02d:%02d", c.hour, c.minute)
}


func (c clock) Add(minutes int ) clock {
	c.hour, c.minute = build(c.hour, c.minute + minutes)
  return c
}

func (c clock) Subtract(minutes int ) clock {
	c.hour, c.minute = build(c.hour, c.minute - minutes)
  return c
}


func build(h, m int) (int, int) {
    h += m / 60
    m %= 60
    if m < 0 {
        m += 60
        h -= 1
    }
    h %= 24
    if h < 0 {
        h += 24
    }
    return h, m
}