code review 5985059: time: panic if UnixNano is out of range

src/pkg/time/time.go

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // Package time provides functionality for measuring and displaying time.
 //
 // The calendrical calculations always assume a Gregorian calendar.
 package time
 
 import "errors"
@@ skipping 746 matching lines @@
 }
 
 // Unix returns t as a Unix time, the number of seconds elapsed
 // since January 1, 1970 UTC.
 func (t Time) Unix() int64 {
         return t.sec + internalToUnix
 }
 
 // UnixNano returns t as a Unix time, the number of nanoseconds elapsed
 // since January 1, 1970 UTC.
+// It is a runtime error to call UnixNano if representing t in nanoseconds
+// would overflow int64. The zero time is outside of the supported range.
 func (t Time) UnixNano() int64 {
-        return (t.sec+internalToUnix)*1e9 + int64(t.nsec)
+        secs := t.sec + internalToUnix
+        nano := secs*1e9 + int64(t.nsec)
+        if (secs < 0) != (nano < 0) {
+                panic("UnixNano: time in nanoseconds overflows int64")
+        }
+        return nano
 }
 
 const timeGobVersion byte = 1
 
 // GobEncode implements the gob.GobEncoder interface.
 func (t Time) GobEncode() ([]byte, error) {
         var offsetMin int16 // minutes east of UTC. -1 is UTC.
 
         if t.Location() == &utcLoc {
                 offsetMin = -1
@@ skipping 199 matching lines @@
                 case utc < start:
                         _, offset, _, _, _ = loc.lookup(start - 1)
                 case utc >= end:
                         _, offset, _, _, _ = loc.lookup(end)
                 }
                 unix -= int64(offset)
         }
 
         return Time{unix + unixToInternal, int32(nsec), loc}
 }