few changes

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 go_import_path: qvl.io/promplot
 go:
-  - 1.9
+  - "1.15"
 os:
   - linux
   - osx

flags/duration.go

@@ -2,6 +2,7 @@ package flags
 
 import (
 	"flag"
+	"fmt"
 	"regexp"
 	"strconv"
 	"time"
@@ -20,8 +21,11 @@ func (d *durationValue) Set(s string) error {
 		s = s[:is[0]] + s[is[1]:] + strconv.FormatFloat(days*24, 'f', 6, 64) + "h"
 	}
 	v, err := time.ParseDuration(s)
+	if err != nil {
+		return fmt.Errorf("failed to parse string time: %v", err)
+	}
 	*d = durationValue(v)
-	return err
+	return nil
 }
 
 func (d *durationValue) String() string {

flags/unixtime.go

@@ -2,6 +2,7 @@ package flags
 
 import (
 	"flag"
+	"fmt"
 	"time"
 )
 
@@ -13,8 +14,11 @@ func (t *unixTime) String() string {
 
 func (t *unixTime) Set(s string) error {
 	parsed, err := time.Parse(time.UnixDate, s)
+	if err != nil {
+		return fmt.Errorf("failed to parse string time: %v", err)
+	}
 	*t = unixTime(parsed)
-	return err
+	return nil
 }
 
 // UnixTime defines a flag for time.Time values formatted as Unix date.

readme.md

@@ -1,6 +1,6 @@
 #  :chart_with_upwards_trend: promplot
 
-[![GoDoc](https://godoc.org/qvl.io/promplot?status.svg)](https://godoc.org/qvl.io/promplot)
+[![GoDoc](https://godoc.org/qvl.io/promplot?status.svg)](https://pkg.go.dev/qvl.io/promplot)
 [![Build Status](https://travis-ci.org/qvl/promplot.svg?branch=master)](https://travis-ci.org/qvl/promplot)
 [![Go Report Card](https://goreportcard.com/badge/qvl.io/promplot)](https://goreportcard.com/report/qvl.io/promplot)