from what i found at lvl21 max with my spreadsheet DO NOT charge anything on a boss. timeshift wants you to think it's good, but a charge takes 1s + .5s GCD, because of that you could get in 1 more AA in during that lost 1s and you get like 11 damage dot for ~8ish seconds.

i do the same, but i start with a charged Time to start. I found that the +3s duration on frost make the dot go on each cast, charged or not
the thing that works better when charging is the aoe on impact, causing frost/slow to multiple mobs. after the initial TS charge, i just AA then skill, without charging anything.