time: add examples for Since, Until, Abs and fix some comments

Change-Id: I33b61629dfabffa15065a14fccdb418bab11350d
Reviewed-on: https://go-review.googlesource.com/c/go/+/623915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/time/example_test.go

@@ -6,6 +6,7 @@ package time_test
 
 import (
 	"fmt"
+	"math"
 	"time"
 )
 
@@ -108,6 +109,39 @@ func ExampleParseDuration() {
 	// There are 1.00e-06 seconds in 1µs.
 }
 
+func ExampleSince() {
+	start := time.Now()
+	expensiveCall()
+	elapsed := time.Since(start)
+	fmt.Printf("The call took %v to run.\n", elapsed)
+}
+
+func ExampleUntil() {
+	futureTime := time.Now().Add(5 * time.Second)
+	durationUntil := time.Until(futureTime)
+	fmt.Printf("Duration until future time: %.0f seconds", math.Ceil(durationUntil.Seconds()))
+	// Output: Duration until future time: 5 seconds
+}
+
+func ExampleDuration_Abs() {
+	positiveDuration := 5 * time.Second
+	negativeDuration := -3 * time.Second
+	minInt64CaseDuration := time.Duration(math.MinInt64)
+
+	absPositive := positiveDuration.Abs()
+	absNegative := negativeDuration.Abs()
+	absSpecial := minInt64CaseDuration.Abs() == time.Duration(math.MaxInt64)
+
+	fmt.Printf("Absolute value of positive duration: %v\n", absPositive)
+	fmt.Printf("Absolute value of negative duration: %v\n", absNegative)
+	fmt.Printf("Absolute value of MinInt64 equal to MaxInt64: %t\n", absSpecial)
+
+	// Output:
+	// Absolute value of positive duration: 5s
+	// Absolute value of negative duration: 3s
+	// Absolute value of MinInt64 equal to MaxInt64: true
+}
+
 func ExampleDuration_Hours() {
 	h, _ := time.ParseDuration("4h30m")
 	fmt.Printf("I've got %.1f hours of work left.", h.Hours())

src/time/time_test.go

@@ -1400,7 +1400,7 @@ var defaultLocTests = []struct {
 	{"Add", func(t1, t2 Time) bool { return t1.Add(Hour).Equal(t2.Add(Hour)) }},
 	{"Sub", func(t1, t2 Time) bool { return t1.Sub(t2) == t2.Sub(t1) }},
 
-	//Original caus for this test case bug 15852
+	// Original cause for this test case bug 15852
 	{"AddDate", func(t1, t2 Time) bool { return t1.AddDate(1991, 9, 3) == t2.AddDate(1991, 9, 3) }},
 
 	{"UTC", func(t1, t2 Time) bool { return t1.UTC() == t2.UTC() }},