Better attempt at an appengine wrapper.

go/src/infra/gae/libs/wrapper/time.go

+// Copyright 2015 The Chromium 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 wrapper
+
+import (
+        "time"
+
+        "golang.org/x/net/context"
+)
+
+// TimeNowFactory is the function signature for factory methods compatible with
+// SetTimeNowFactory.
+type TimeNowFactory func(context.Context) time.Time
+
+// GetTimeNow gets the current time.Time from the context. If the context has no
+// time factory set with SetTimeNowFactory, it returns time.Now().
+func GetTimeNow(c context.Context) time.Time {
+        if f, ok := c.Value(timeNowKey).(TimeNowFactory); ok && f != nil {
+                return f(c)
+        }
+        return time.Now()
+}
+
+// SetTimeNowFactory sets the function to produce time.Time instances, as
+// returned by the GetTimeNow method.
+func SetTimeNowFactory(c context.Context, tnf TimeNowFactory) context.Context {
+        return context.WithValue(c, timeNowKey, tnf)
+}
+
+// SetTimeNow sets a pointer to the current time.Time object in the context.
+// Useful for testing with a quick mock. This is just a shorthand
+// SetTimeNowFactory invocation to set a factory which always returns the same
+// object.
+//
+// Note the pointer asymmetry with GetTimeNow. This is done intentionally to
+// allow you to modify the pointed-to-time to be able to manually monkey with
+// the clock in a test. Otherwise this would essentially freeze time in place.
+// If you pass 'nil', this will set the context back to returning time.Now().
+func SetTimeNow(c context.Context, t *time.Time) context.Context {
+        if t == nil {
+                return SetTimeNowFactory(c, nil)
+        }
+        return SetTimeNowFactory(c, func(context.Context) time.Time { return *t })
+}