Add context-aware "time" library wrapper.

go/src/infra/libs/clock/clockflag/time.go

Index: go/src/infra/libs/clock/clockflag/time.go
diff --git a/go/src/infra/libs/clock/clockflag/time.go b/go/src/infra/libs/clock/clockflag/time.go
new file mode 100644
index 0000000000000000000000000000000000000000..3065183950ca4c550c54434a5d02bffb76175b74
--- /dev/null
+++ b/go/src/infra/libs/clock/clockflag/time.go
@@ -0,0 +1,59 @@
+// Copyright 2014 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 clockflag
+
+import (
+	"encoding/json"
+	"flag"
+	"time"
+)
+
+// Time is a flag- and JSON-compatible Time which parses from RFC3339 strings.
+type Time time.Time
+
+var _ interface {
+	flag.Value
+	json.Unmarshaler
+	json.Marshaler
+} = (*Time)(nil)
+
+// Time returns the Time value associated with this Time.
+func (t Time) Time() time.Time {
+	return time.Time(t)
+}
+
+// Set implements flag.Value.
+func (t *Time) Set(value string) error {
+	timeValue, err := time.Parse(time.RFC3339Nano, value)
+	if err != nil {
+		return err
+	}
+	*t = Time(timeValue.UTC())
+	return nil
+}
+
+func (t *Time) String() string {
+	return time.Time(*t).String()
+}
+
+// UnmarshalJSON implements json.Unmarshaler.
+//
+// Unmarshals a JSON entry into the underlying type. The entry is expected to
+// contain a string corresponding to one of the enum's keys.
+func (t *Time) UnmarshalJSON(data []byte) error {
+	var value time.Time
+	if err := value.UnmarshalJSON(data); err != nil {
+		return err
+	}
+	*t = Time(value.UTC())
+	return nil
+}
+
+// MarshalJSON implements json.Marshaler.
+//
+// Marshals a Time into an RFC3339 time string.
+func (t Time) MarshalJSON() ([]byte, error) {
+	return t.Time().UTC().MarshalJSON()
+}