Remaining cast time as a tag?

[Dangergoppel]

(Might be obvious to most, but hey - I'm a newb at this, bear with me!)
I'm sure this is possible, but I've spent this whole morning trying to figure it out. Also, which event(s) is/are the best for updating this? I've gotten so far as to having the remaining duration without updates, and with like 100 decimals.

Any help appreciated.