SpecSchedule.Next: use time.Truncate instead of time.Date

Add tests to verify nightly jobs work as expected.
This fixes an infinite loop when clock falls back for DST

spec.go

@@ -1,8 +1,6 @@
 package cron
 
-import (
-	"time"
-)
+import "time"
 
 // SpecSchedule specifies a duty cycle (to the second granularity), based on a
 // traditional crontab specification. It is computed initially and stored as bit sets.
@@ -110,7 +108,7 @@ WRAP:
 	for 1<<uint(t.Hour())&s.Hour == 0 {
 		if !added {
 			added = true
-			t = time.Date(t.Year(), t.Month(), t.Day(), t.Hour(), 0, 0, 0, t.Location())
+			t = t.Truncate(time.Hour)
 		}
 		t = t.Add(1 * time.Hour)
 
@@ -122,7 +120,7 @@ WRAP:
 	for 1<<uint(t.Minute())&s.Minute == 0 {
 		if !added {
 			added = true
-			t = time.Date(t.Year(), t.Month(), t.Day(), t.Hour(), t.Minute(), 0, 0, t.Location())
+			t = t.Truncate(time.Minute)
 		}
 		t = t.Add(1 * time.Minute)
 
@@ -134,7 +132,7 @@ WRAP:
 	for 1<<uint(t.Second())&s.Second == 0 {
 		if !added {
 			added = true
-			t = time.Date(t.Year(), t.Month(), t.Day(), t.Hour(), t.Minute(), t.Second(), 0, t.Location())
+			t = t.Truncate(time.Second)
 		}
 		t = t.Add(1 * time.Second)
 

spec_test.go

@@ -109,13 +109,44 @@ func TestNext(t *testing.T) {
 		// Leap year
 		{"Mon Jul 9 23:35 2012", "0 0 0 29 Feb ?", "Mon Feb 29 00:00 2016"},
 
-		// Daylight savings time EST -> EDT
+		// Daylight savings time 2am EST (-5) -> 3am EDT (-4)
 		{"2012-03-11T00:00:00-0500", "0 30 2 11 Mar ?", "2013-03-11T02:30:00-0400"},
 
-		// Daylight savings time EDT -> EST
+		// hourly job
+		{"2012-03-11T00:00:00-0500", "0 0 * * * ?", "2012-03-11T01:00:00-0500"},
+		{"2012-03-11T01:00:00-0500", "0 0 * * * ?", "2012-03-11T03:00:00-0400"},
+		{"2012-03-11T03:00:00-0400", "0 0 * * * ?", "2012-03-11T04:00:00-0400"},
+		{"2012-03-11T04:00:00-0400", "0 0 * * * ?", "2012-03-11T05:00:00-0400"},
+
+		// 1am nightly job
+		{"2012-03-11T00:00:00-0500", "0 0 1 * * ?", "2012-03-11T01:00:00-0500"},
+		{"2012-03-11T01:00:00-0500", "0 0 1 * * ?", "2012-03-12T01:00:00-0400"},
+
+		// 2am nightly job (skipped)
+		{"2012-03-11T00:00:00-0500", "0 0 2 * * ?", "2012-03-12T02:00:00-0400"},
+
+		// Daylight savings time 2am EDT (-4) => 1am EST (-5)
 		{"2012-11-04T00:00:00-0400", "0 30 2 04 Nov ?", "2012-11-04T02:30:00-0500"},
 		{"2012-11-04T01:45:00-0400", "0 30 1 04 Nov ?", "2012-11-04T01:30:00-0500"},
 
+		// hourly job
+		{"2012-11-04T00:00:00-0400", "0 0 * * * ?", "2012-11-04T01:00:00-0400"},
+		{"2012-11-04T01:00:00-0400", "0 0 * * * ?", "2012-11-04T01:00:00-0500"},
+		{"2012-11-04T01:00:00-0500", "0 0 * * * ?", "2012-11-04T02:00:00-0500"},
+
+		// 1am nightly job (runs twice)
+		{"2012-11-04T00:00:00-0400", "0 0 1 * * ?", "2012-11-04T01:00:00-0400"},
+		{"2012-11-04T01:00:00-0400", "0 0 1 * * ?", "2012-11-04T01:00:00-0500"},
+		{"2012-11-04T01:00:00-0500", "0 0 1 * * ?", "2012-11-05T01:00:00-0500"},
+
+		// 2am nightly job
+		{"2012-11-04T00:00:00-0400", "0 0 2 * * ?", "2012-11-04T02:00:00-0500"},
+		{"2012-11-04T02:00:00-0500", "0 0 2 * * ?", "2012-11-05T02:00:00-0500"},
+
+		// 3am nightly job
+		{"2012-11-04T00:00:00-0400", "0 0 3 * * ?", "2012-11-04T03:00:00-0500"},
+		{"2012-11-04T03:00:00-0500", "0 0 3 * * ?", "2012-11-05T03:00:00-0500"},
+
 		// Unsatisfiable
 		{"Mon Jul 9 23:35 2012", "0 0 0 30 Feb ?", ""},
 		{"Mon Jul 9 23:35 2012", "0 0 0 31 Apr ?", ""},