Use RFC3339 in UTC as the default time format for log messages

This ensures that times that are logged in messages can be deserialised
by the encoding/json package irrespective of the timezone that the time
is in when logged; currently if the time is not in UTC, the ISO8601
format that is used is not accepted. Also provide a non-UTC formatter
that can be used where the timezone is important.

CHANGELOG.md

@@ -2,6 +2,7 @@
 Changelog for ecszap
 
 ## unreleased
+* Use RFC3339 in UTC as the default time format for log messages [pull#84](https://github.com/elastic/ecs-logging-go-zap/pull/84)
 
 ## 1.0.2
 * Updated zap to v1.24.0 [pull#50](https://github.com/elastic/ecs-logging-go-zap/pull/50)

encoder.go

@@ -49,6 +49,40 @@ func ShortCallerEncoder(c zapcore.EntryCaller, enc zapcore.PrimitiveArrayEncoder
 	encodeCaller(&caller{c, false}, enc)
 }
 
+// RFC3339TimeEncoder serializes a time.Time to an RFC3339-formatted string
+// with millisecond precision.
+//
+// If enc supports AppendTimeLayout(t time.Time,layout string), it's used
+// instead of appending a pre-formatted string value.
+func RFC3339TimeEncoder(t time.Time, enc zapcore.PrimitiveArrayEncoder) {
+	t = t.In(time.UTC)
+	type appendTimeEncoder interface {
+		AppendTimeLayout(time.Time, string)
+	}
+	// Use a custom RFC3339 layout with obligatory millisecond
+	// precision rather than the second-resolution or optional
+	// nano-second resolution that is provided in the time
+	// package.
+	const rfc3339millis = "2006-01-02T15:04:05.000Z07:00"
+	if enc, ok := enc.(appendTimeEncoder); ok {
+		enc.AppendTimeLayout(t, rfc3339millis)
+		return
+	}
+
+	enc.AppendString(t.Format(rfc3339millis))
+}
+
+// RFC3339UTCTimeEncoder serializes a time.Time to an RFC3339-formatted string
+// with millisecond precision in UTC.
+//
+// If enc supports AppendTimeLayout(t time.Time,layout string), it's used
+// instead of appending a pre-formatted string value.
+//
+// RFC3339UTCTimeEncoder is the default time encoder used by EncoderConfig.
+func RFC3339UTCTimeEncoder(t time.Time, enc zapcore.PrimitiveArrayEncoder) {
+	RFC3339TimeEncoder(t.In(time.UTC), enc)
+}
+
 // UnmarshalText creates a CallerEncoder function,
 // `full` is unmarshalled to FullCallerEncoder,
 // defaults to ShortCallerEncoder,

encoder_config.go

@@ -34,7 +34,7 @@ var (
 	messageKey    = "message"
 	stackTraceKey = "log.origin.stack_trace"
 	timeKey       = "@timestamp"
-	encodeTime    = zapcore.ISO8601TimeEncoder
+	encodeTime    = RFC3339UTCTimeEncoder
 )
 
 // EncoderConfig exports all non ECS related configurable settings.