Improve start parsing

client/offset.go

@@ -2,18 +2,44 @@ package client
 
 import (
 	"fmt"
+	"strconv"
+	"strings"
+	"time"
 
 	"github.com/Shopify/sarama"
 )
 
-// ParseStart returns a start offset parsed from s.
+const dateTimeLayout string = "2006-01-02 15:04:05"
+
+// StartHelp returns a message describing valid start values.
+func StartHelp() string {
+	var b strings.Builder
+	fmt.Fprint(&b, "* available start values :\n")
+	fmt.Fprintf(&b, "newest : start at log head offset\n")
+	fmt.Fprintf(&b, "oldest : start at the oldest available offset\n")
+	fmt.Fprintf(&b, "YYYY-mm-dd HH:MM:SS : start at this date (UTC)\n")
+	fmt.Fprintf(&b, "<epoch in milliseconds> : start at this epoch")
+	return b.String()
+}
+
+// ParseStart returns a start time parsed from s.
 func ParseStart(s string) (int64, error) {
 	switch s {
 	case "oldest":
 		return sarama.OffsetOldest, nil
 	case "newest":
 		return sarama.OffsetNewest, nil
 	default:
+		t, err := time.Parse(dateTimeLayout, s)
+		if err == nil {
+			return t.UnixNano() / int64(time.Millisecond), nil
+		}
+
+		e, err := strconv.ParseInt(s, 10, 64)
+		if err == nil {
+			return e, nil
+		}
+
 		return 0, fmt.Errorf("invalid start offset %q", s)
 	}
 }