Avoid having to parse strings

app/helpers/time_helper.rb

@@ -1,6 +1,6 @@
 module TimeHelper
   def local_datetime_tag(datetime, style: :time, **attributes)
     # Render empty space to ensure it takes height until the local time is loaded via JS
-    tag.time " ".html_safe, **attributes, datetime: datetime.iso8601, data: { local_time_target: style, action: "turbo:morph-element->local-time#refreshTarget" }
+    tag.time " ".html_safe, **attributes, datetime: datetime.to_i, data: { local_time_target: style, action: "turbo:morph-element->local-time#refreshTarget" }
   end
 end

app/javascript/controllers/local_time_controller.js

@@ -89,7 +89,7 @@ export default class extends Controller {
   }
 
   #formatTime(formatter, target) {
-    const dt = new Date(target.getAttribute("datetime"))
+    const dt = new Date(parseInt(target.getAttribute("datetime"))*1000)
     target.innerHTML = formatter.format(dt)
     target.title = this.datetimeFormatter.format(dt)
   }