diff --git a/template/template.go b/template/template.go
index fc388be5dd..dcb6e280ed 100644
--- a/template/template.go
+++ b/template/template.go
@@ -339,6 +339,14 @@ var DefaultFuncs = FuncMap{
 	"mustToDate": func(layout, s string) (time.Time, error) {
 		return time.ParseInLocation(layout, s, time.UTC)
 	},
+	// addDuration parses ds as a Go duration, adds it to t, and returns the resulting Unix milliseconds.
+	"addDuration": func(t time.Time, ds string) (int64, error) {
+		d, err := time.ParseDuration(ds)
+		if err != nil {
+			return 0, err
+		}
+		return t.Add(d).UnixMilli(), nil
+	},
 	"toJson": func(v any) (string, error) {
 		bytes, err := json.Marshal(v)
 		if err != nil {
diff --git a/template/template_test.go b/template/template_test.go
index 9c884ea77d..31823e5ed1 100644
--- a/template/template_test.go
+++ b/template/template_test.go
@@ -902,6 +902,16 @@ func TestTemplateFuncs(t *testing.T) {
 		title:  "Template using mustToDate with invalid input returns error",
 		in:     `{{ mustToDate "2006-01-02" "not-a-date" }}`,
 		expErr: `template: :1:3: executing "" at <mustToDate "2006-01-02" "not-a-date">: error calling mustToDate: parsing time "not-a-date" as "2006-01-02": cannot parse "not-a-date" as "2006"`,
+	}, {
+		title: "Template using addDuration with valid input",
+		in:    `{{ addDuration . "90m" }}`,
+		data:  time.Unix(0, 0).UTC(),
+		exp:   "5400000",
+	}, {
+		title:  "Template using addDuration with invalid input returns error",
+		in:     `{{ addDuration . "not-a-duration" }}`,
+		data:   time.Unix(0, 0).UTC(),
+		expErr: `template: :1:3: executing "" at <addDuration . "not-a-duration">: error calling addDuration: time: invalid duration "not-a-duration"`,
 	}} {
 		t.Run(tc.title, func(t *testing.T) {
 			wg := sync.WaitGroup{}