allow custom time format string in TextFormatter

- fixes examples in README.md that incorrectly state usage of
  RFC3339Nano format instead of RFC3339

README.md

@@ -37,11 +37,13 @@ attached, the output is compatible with the
 [logfmt](http://godoc.org/github.com/kr/logfmt) format:
 
 ```text
-time="2014-04-20 15:36:23.830442383 -0400 EDT" level="info" msg="A group of walrus emerges from the ocean" animal="walrus" size=10
+time="2015-03-26T01:27:38-04:00" level=debug msg="Started observing beach" animal=walrus number=8
-time="2014-04-20 15:36:23.830584199 -0400 EDT" level="warning" msg="The group's number increased tremendously!" omg=true number=122
+time="2015-03-26T01:27:38-04:00" level=info msg="A group of walrus emerges from the ocean" animal=walrus size=10
-time="2014-04-20 15:36:23.830596521 -0400 EDT" level="info" msg="A giant walrus appears!" animal="walrus" size=10
+time="2015-03-26T01:27:38-04:00" level=warning msg="The group's number increased tremendously!" number=122 omg=true
-time="2014-04-20 15:36:23.830611837 -0400 EDT" level="info" msg="Tremendously sized cow enters the ocean." animal="walrus" size=9
+time="2015-03-26T01:27:38-04:00" level=debug msg="Temperature changes" temperature=-4
-time="2014-04-20 15:36:23.830626464 -0400 EDT" level="fatal" msg="The ice breaks!" omg=true number=100
+time="2015-03-26T01:27:38-04:00" level=panic msg="It's over 9000!" animal=orca size=9009
+time="2015-03-26T01:27:38-04:00" level=fatal msg="The ice breaks!" err=&{0x2082280c0 map[animal:orca size:9009] 2015-03-26 01:27:38.441574009 -0400 EDT panic It's over 9000!} number=100 omg=true
+exit status 1
 ```
 
 #### Example

text_formatter.go

@@ -20,6 +20,7 @@ const (
 var (
 	baseTimestamp          time.Time
 	isTerminal             bool
+	defaultTimestampFormat = time.RFC3339
 )
 
 func init() {
@@ -46,6 +47,9 @@ type TextFormatter struct {
 	// the time passed since beginning of execution.
 	FullTimestamp bool
 
+	// Timestamp format to use for display, if a full timestamp is printed
+	TimestampFormat string
+
 	// The fields are sorted by default for a consistent output. For applications
 	// that log extremely frequently and don't use the JSON formatter this may not
 	// be desired.
@@ -68,11 +72,14 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 
 	isColored := (f.ForceColors || isTerminal) && !f.DisableColors
 
+	if f.TimestampFormat == "" {
+		f.TimestampFormat = defaultTimestampFormat
+	}
 	if isColored {
 		f.printColored(b, entry, keys)
 	} else {
 		if !f.DisableTimestamp {
-			f.appendKeyValue(b, "time", entry.Time.Format(time.RFC3339))
+			f.appendKeyValue(b, "time", entry.Time.Format(f.TimestampFormat))
 		}
 		f.appendKeyValue(b, "level", entry.Level.String())
 		f.appendKeyValue(b, "msg", entry.Message)
@@ -103,7 +110,7 @@ func (f *TextFormatter) printColored(b *bytes.Buffer, entry *Entry, keys []strin
 	if !f.FullTimestamp {
 		fmt.Fprintf(b, "\x1b[%dm%s\x1b[0m[%04d] %-44s ", levelColor, levelText, miniTS(), entry.Message)
 	} else {
-		fmt.Fprintf(b, "\x1b[%dm%s\x1b[0m[%s] %-44s ", levelColor, levelText, entry.Time.Format(time.RFC3339), entry.Message)
+		fmt.Fprintf(b, "\x1b[%dm%s\x1b[0m[%s] %-44s ", levelColor, levelText, entry.Time.Format(f.TimestampFormat), entry.Message)
 	}
 	for _, k := range keys {
 		v := entry.Data[k]

text_formatter_test.go

@@ -3,8 +3,8 @@ package logrus
 import (
 	"bytes"
 	"errors"
-
 	"testing"
+	"time"
 )
 
 func TestQuoting(t *testing.T) {
@@ -33,5 +33,29 @@ func TestQuoting(t *testing.T) {
 	checkQuoting(true, errors.New("invalid argument"))
 }
 
+func TestTimestampFormat(t *testing.T) {
+	checkTimeStr := func(format string) {
+		customFormatter := &TextFormatter{DisableColors: true, TimestampFormat: format}
+		customStr, _ := customFormatter.Format(WithField("test", "test"))
+		timeStart := bytes.Index(customStr, ([]byte)("time="))
+		timeEnd := bytes.Index(customStr, ([]byte)("level="))
+		timeStr := customStr[timeStart+5 : timeEnd-1]
+		if timeStr[0] == '"' && timeStr[len(timeStr)-1] == '"' {
+			timeStr = timeStr[1 : len(timeStr)-1]
+		}
+		if format == "" {
+			format = time.RFC3339
+		}
+		_, e := time.Parse(format, (string)(timeStr))
+		if e != nil {
+			t.Errorf("time string \"%s\" did not match provided time format \"%s\": %s", timeStr, format, e)
+		}
+	}
+
+	checkTimeStr("2006-01-02T15:04:05.000000000Z07:00")
+	checkTimeStr("Mon Jan _2 15:04:05 2006")
+	checkTimeStr("")
+}
+
 // TODO add tests for sorting etc., this requires a parser for the text
 // formatter output.