Refactored Clock to only store minutes...

add functions to return hours and minutes
  benchmarked passing results

bench0

@@ -0,0 +1,4 @@
+PASS
+BenchmarkAddMinutes-12  	30000000	        45.7 ns/op
+BenchmarkCreateClocks-12	30000000	        52.1 ns/op
+ok  	exercism/go/clock	3.043s

cases_test.go

@@ -22,6 +22,8 @@ var timeTests = []struct {
 	{1, -40, "00:20"},    // negative minutes
 	{1, -160, "22:20"},   // negative minutes roll over
 	{-25, -160, "20:20"}, // negative hour and minutes both roll over
+	{-25, -1500, "22:00"},
+	{-49, -3000, "21:00"},
 }
 
 // Test adding and subtracting minutes.

clock.go

@@ -4,38 +4,54 @@ import "fmt"
 
 const (
 	testVersion    = 3
-	minutesPerHour = 60
 	hoursPerDay    = 24
+	minutesPerHour = 60
+	minutesPerDay  = 1440
 )
 
+// A Clock stores a minute value representing a time of day.
 type Clock struct {
-	hour    int
 	minutes int
 }
 
-func New(hour, minute int) Clock {
-	h, m := adjustTime(hour, minute)
-	return Clock{h, m}
-}
-
+// String returns an hour:minute representation of the Clock.
 func (c Clock) String() string {
-	return fmt.Sprintf("%02d:%02d", c.hour, c.minutes)
+	return fmt.Sprintf("%02d:%02d", c.hour(), c.mins())
 }
 
+// Add returns a Clock adjusted by the number of minutes specified.
 func (c Clock) Add(minutes int) Clock {
-	min := c.minutes + minutes
-	c.hour, c.minutes = adjustTime(c.hour, min)
+	c.minutes = parseTime(0, c.minutes+minutes)
 	return c
 }
 
-func adjustTime(h, m int) (int, int) {
-	h = (h + (m / minutesPerHour)) % hoursPerDay
-	if h < 0 {
-		h += hoursPerDay
-	}
-	m = m % minutesPerHour
-	if m < 0 {
-		m += minutesPerHour
+// hour computes the hourly portion of the Clock.
+func (c Clock) hour() int {
+	return c.minutes / minutesPerHour
+}
+
+// mins computes the non-hourly remainder of the Clock.
+func (c Clock) mins() int {
+	return c.minutes % minutesPerHour
+}
+
+// New returns a Clock constructed from the given hour and minute values.
+func New(hour, minutes int) Clock {
+	return Clock{parseTime(hour, minutes)}
+}
+
+// parseTime computes the number of minutes that represent the time.
+func parseTime(hour, minutes int) int {
+	// normalize hours that are out of range [0-23]
+	hour = hour % hoursPerDay
+	// normalize any out of range values [0-1439]
+	minutes = minutes % minutesPerDay
+	// compute the total minutes
+	total := hour * minutesPerHour
+	total += minutes
+	// normalize any negative minute values
+	if total < 0 {
+		total += minutesPerDay
 	}
-	return h, m
+	return total
 }