diff --git a/authority/provisioner/duration.go b/authority/provisioner/duration.go
index 68ffbb0a..d18a81e9 100644
--- a/authority/provisioner/duration.go
+++ b/authority/provisioner/duration.go
@@ -12,6 +12,16 @@ type Duration struct {
 	time.Duration
 }
 
+// NewDuration parses a duration string and returns a Duration type or and error
+// if the given string is not a duration.
+func NewDuration(s string) (*Duration, error) {
+	d, err := time.ParseDuration(s)
+	if err != nil {
+		return nil, errors.Wrapf(err, "error parsing %s as duration", s)
+	}
+	return &Duration{Duration: d}, nil
+}
+
 // MarshalJSON parses a duration string and sets it to the duration.
 //
 // A duration string is a possibly signed sequence of decimal numbers, each with
@@ -29,7 +39,7 @@ func (d *Duration) MarshalJSON() ([]byte, error) {
 func (d *Duration) UnmarshalJSON(data []byte) (err error) {
 	var (
 		s  string
-		_d time.Duration
+		dd time.Duration
 	)
 	if d == nil {
 		return errors.New("duration cannot be nil")
@@ -37,10 +47,10 @@ func (d *Duration) UnmarshalJSON(data []byte) (err error) {
 	if err = json.Unmarshal(data, &s); err != nil {
 		return errors.Wrapf(err, "error unmarshaling %s", data)
 	}
-	if _d, err = time.ParseDuration(s); err != nil {
+	if dd, err = time.ParseDuration(s); err != nil {
 		return errors.Wrapf(err, "error parsing %s as duration", s)
 	}
-	d.Duration = _d
+	d.Duration = dd
 	return
 }
 
diff --git a/authority/provisioner/duration_test.go b/authority/provisioner/duration_test.go
index faf5e7f4..828064cc 100644
--- a/authority/provisioner/duration_test.go
+++ b/authority/provisioner/duration_test.go
@@ -6,6 +6,35 @@ import (
 	"time"
 )
 
+func TestNewDuration(t *testing.T) {
+	type args struct {
+		s string
+	}
+	tests := []struct {
+		name    string
+		args    args
+		want    *Duration
+		wantErr bool
+	}{
+		{"ok", args{"1h2m3s"}, &Duration{Duration: 3723 * time.Second}, false},
+		{"fail empty", args{""}, nil, true},
+		{"fail number", args{"123"}, nil, true},
+		{"fail string", args{"1hour"}, nil, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := NewDuration(tt.args.s)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("NewDuration() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("NewDuration() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
 func TestDuration_UnmarshalJSON(t *testing.T) {
 	type args struct {
 		data []byte