Add WithTimeSource Option

Allow passing current time source.

logger.go

@@ -51,6 +51,9 @@ type Logger struct {
 	addStack zapcore.LevelEnabler
 
 	callerSkip int
+
+	// now returns current time, default is time.Now.
+	now func() time.Time
 }
 
 // New constructs a new Logger from the provided zapcore.Core and Options. If
@@ -71,6 +74,7 @@ func New(core zapcore.Core, options ...Option) *Logger {
 		core:        core,
 		errorOutput: zapcore.Lock(os.Stderr),
 		addStack:    zapcore.FatalLevel + 1,
+		now:         time.Now,
 	}
 	return log.WithOptions(options...)
 }
@@ -270,7 +274,7 @@ func (log *Logger) check(lvl zapcore.Level, msg string) *zapcore.CheckedEntry {
 	// log message will actually be written somewhere.
 	ent := zapcore.Entry{
 		LoggerName: log.name,
-		Time:       time.Now(),
+		Time:       log.now(),
 		Level:      lvl,
 		Message:    msg,
 	}
@@ -307,7 +311,7 @@ func (log *Logger) check(lvl zapcore.Level, msg string) *zapcore.CheckedEntry {
 	if log.addCaller {
 		frame, defined := getCallerFrame(log.callerSkip + callerSkipOffset)
 		if !defined {
-			fmt.Fprintf(log.errorOutput, "%v Logger.check error: failed to get caller\n", time.Now().UTC())
+			fmt.Fprintf(log.errorOutput, "%v Logger.check error: failed to get caller\n", log.now().UTC())
 			log.errorOutput.Sync()
 		}
 

options.go

@@ -22,6 +22,7 @@ package zap
 
 import (
 	"fmt"
+	"time"
 
 	"go.uber.org/zap/zapcore"
 )
@@ -138,3 +139,11 @@ func OnFatal(action zapcore.CheckWriteAction) Option {
 		log.onFatal = action
 	})
 }
+
+// WithTimeSource configures the Logger to get current time from provided
+// function when creating entry.
+func WithTimeSource(now func() time.Time) Option {
+	return optionFunc(func(log *Logger) {
+		log.now = now
+	})
+}

time_source_test.go

@@ -0,0 +1,20 @@
+package zap
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/zap/zaptest/observer"
+)
+
+func TestWithTimeSource(t *testing.T) {
+	date := time.Date(2077, 1, 23, 10, 15, 13, 441, time.UTC)
+	now := func() time.Time { return date }
+	withLogger(t, DebugLevel, []Option{WithTimeSource(now)}, func(log *Logger, logs *observer.ObservedLogs) {
+		log.Info("")
+		require.Equal(t, 1, logs.Len(), "Expected only one log entry to be written.")
+		assert.Equal(t, date, logs.All()[0].Entry.Time, "Unexpected entry time.")
+	})
+}