Calculate offset to a timepicker and use the new time as trigger for an automation

so i’m trying to create yet another alarm clock. i just got one of those yeelight bedsidelamp that i would like to integrate into my alarm routine, but without adding a second timepicker.

the light has a script for a sunrise simulation which takes about 30 minutes to reach full brightness, so that is working. my alarm starts at a specified time with a time picker from the helper in HA. now i want the light to start 15min before the time set via the picker. so the picker is st to 7am which means the light should start 6.45am. could someone please help me out? i’m pretty new to all the homeassistant stuff! thanks