scrobblerlog: Allow configuring fallback time zone

Fixes #6

internal/backends/scrobblerlog/scrobblerlog.go

@@ -18,6 +18,7 @@ package scrobblerlog
 
 import (
 	"bufio"
+	"fmt"
 	"os"
 	"sort"
 	"strings"
@@ -34,6 +35,7 @@ type ScrobblerLogBackend struct {
 	includeSkipped bool
 	append         bool
 	file           *os.File
+	timezone       *time.Location
 	log            scrobblerlog.ScrobblerLog
 }
 
@@ -53,6 +55,10 @@ func (b *ScrobblerLogBackend) Options() []models.BackendOption {
 		Label:   i18n.Tr("Append to file"),
 		Type:    models.Bool,
 		Default: "true",
+	}, {
+		Name:  "time-zone",
+		Label: i18n.Tr("Specify a time zone for the listen timestamps"),
+		Type:  models.String,
 	}}
 }
 
@@ -60,6 +66,14 @@ func (b *ScrobblerLogBackend) InitConfig(config *config.ServiceConfig) error {
 	b.filePath = config.GetString("file-path")
 	b.includeSkipped = config.GetBool("include-skipped", false)
 	b.append = config.GetBool("append", true)
+	timezone := config.GetString("time-zone")
+	if timezone != "" {
+		location, err := time.LoadLocation(timezone)
+		if err != nil {
+			return fmt.Errorf("Invalid time-zone %q: %w", timezone, err)
+		}
+		b.log.FallbackTimezone = location
+	}
 	b.log = scrobblerlog.ScrobblerLog{
 		TZ:     scrobblerlog.TZ_UTC,
 		Client: "Rockbox unknown $Revision$",

internal/backends/scrobblerlog/scrobblerlog_test.go

@@ -33,3 +33,13 @@ func TestInitConfig(t *testing.T) {
 	err := backend.InitConfig(&service)
 	assert.NoError(t, err)
 }
+
+func TestInitConfigInvalidTimezone(t *testing.T) {
+	c := viper.New()
+	configuredTimezone := "Invalid/Timezone"
+	c.Set("time-zone", configuredTimezone)
+	service := config.NewServiceConfig("test", c)
+	backend := scrobblerlog.ScrobblerLogBackend{}
+	err := backend.InitConfig(&service)
+	assert.ErrorContains(t, err, `Invalid time-zone "Invalid/Timezone"`)
+}