time: fix handling of Jan 1 1601, 1201, 801, ...

R=r
CC=hurtonm
https://golang.org/cl/164074

src/pkg/time/time.go

@@ -104,15 +104,15 @@ func SecondsToUTC(sec int64) *Time {
 		n := -day/daysPer400Years + 1;
 		year -= 400 * n;
 		day += daysPer400Years * n;
-	} else {
-		// Cut off 400 year cycles.
-		n := day / daysPer400Years;
-		year += 400 * n;
-		day -= daysPer400Years * n;
 	}
 
+	// Cut off 400 year cycles.
+	n := day / daysPer400Years;
+	year += 400 * n;
+	day -= daysPer400Years * n;
+
 	// Cut off 100-year cycles
-	n := day / daysPer100Years;
+	n = day / daysPer100Years;
 	year += 100 * n;
 	day -= daysPer100Years * n;
 

src/pkg/time/time_test.go

@@ -26,6 +26,7 @@ var utctests = []TimeTest{
 	TimeTest{0, Time{1970, 1, 1, 0, 0, 0, Thursday, 0, "UTC"}},
 	TimeTest{1221681866, Time{2008, 9, 17, 20, 4, 26, Wednesday, 0, "UTC"}},
 	TimeTest{-1221681866, Time{1931, 4, 16, 3, 55, 34, Thursday, 0, "UTC"}},
+	TimeTest{-11644473600, Time{1601, 1, 1, 0, 0, 0, Monday, 0, "UTC"}},
 	TimeTest{1e18, Time{31688740476, 10, 23, 1, 46, 40, Friday, 0, "UTC"}},
 	TimeTest{-1e18, Time{-31688736537, 3, 10, 22, 13, 20, Tuesday, 0, "UTC"}},
 	TimeTest{0x7fffffffffffffff, Time{292277026596, 12, 4, 15, 30, 7, Sunday, 0, "UTC"}},