So when do you start the 12hr cooking cycle?

Very good question! There are several approaches.

The first is, as you said, start it 12 hours before you want to eat, which doesn't always work great for some schedules / cook times, though you can often fudge the time a few hours in either direction. So maybe seal the steak up and have it ready, then drop it in the waterbath right when you wake up the next morning. That timing might be close enough for you.

Another option is to cook the steak for the 12 hours, then pull it out and chill it in an ice bath. You could then refrigerate it before bringing it back up to temperature in a few days when you get home from work.

Ok thank you for the reply. I can try either. What about cooking partway and then cook the rest the following day? Have you ever done that? Like for instance, a German friend cooks their schweinebraten for several hours, then puts in back in the fridge and (re)cooks it again the next day. So I thought hey this might work for the skirt steak.

I think doing a partial cook would work as well. Just make sure when you are cooling it off you use an ice bath to reduce the temperature as quickly as possible.