Hi @Daisuke
Have you seen this tutorial yet?
You could enter your time on a web page and then on a button click script have it POST to the core via a Spark function when you hit a “start” button on the web page.
Does “15:00” in your example above mean 15 minutes and zero seconds or does it mean 15 hours and zero minutes or does it mean at 15:00 real-time, which we would call 3:00 pm in the US?
Are you making an alarm clock?