Thank you for the suggestions and tips. This actually made me solve my “problem”.
I was planning to use the alarm time as end time in Piggy. Because then, to change end time I can simply change alarm time from the homey app. (I don’t like to use voice assistance, and prefer to keep Google out as much as possible).
Since I have no idea how to program in HomeyScrip I didn’t think of this as a solution before.
But your suggestion got me investigating, and after some searching I found this gem on the dutch part of the forum:
https://community.homey.app/t/wekker-tijd-uitlezen/74488/7
So I shamelessly copied this script and translated it to my needs (thank you @Hamster)
I made a flow to run this scrip at periodic intervals as I don’t know how you could make it trigger based. And this works perfectly, and now I can use HomeyAlarm time as charging end time in Piggy 
