add expr-lang.org support (use with -e)

cmd/config.go

@@ -93,6 +93,7 @@ type Config struct {
 	Examples    bool   `koanf:"examples"`
 	Unit        bool   `koanf:"unit"`
 	Format      string `koanf:"format"`
+	Expr        string `koanf:"expr"`
 	Args        []string
 	Output      io.Writer
 	Mode        int
@@ -121,8 +122,9 @@ func InitConfig(output io.Writer) (*Config, error) {
 	flagset.BoolP("diff", "d", false, "diff two timestamps")
 	flagset.BoolP("add", "a", false, "add two timestamps")
 	flagset.BoolP("unit", "u", false, "add unit to diff outputs")
-	flagset.BoolP("examples", "e", false, "show examples of supported inputs")
+	flagset.BoolP("examples", "E", false, "show examples of supported inputs")
 	flagset.StringP("format", "f", "", "format to print timestamps or diffs")
+	flagset.StringP("expr", "e", "", "run expr program")
 
 	if err := flagset.Parse(os.Args[1:]); err != nil {
 		return nil, fmt.Errorf("failed to parse program arguments: %w", err)
@@ -161,6 +163,11 @@ func InitConfig(output io.Writer) (*Config, error) {
 		os.Exit(0)
 	}
 
+	if conf.Expr != "" {
+		// ignore args etc
+		return conf, nil
+	}
+
 	// args are timestamps
 	if len(flagset.Args()) == 0 {
 		return nil, errors.New("no timestamp argument[s] specified.\n" + Usage)