Somewhat off topic, but I don't like the implication that because Until covers something, there can't be other similar durations.
Faerie Magic especially seems to have a wide-ranging 'whatever you want it to be' duration in Until, and Spell Timing is even worse... but I'd hate to think that they destroy the design space for other, more limited durations.
Looking at the books, though, there are definitely overlapping durations, and I like that. For example... Spell Timing provides you with the While duration, which is similar to concentration and lasts for as long as you do something specific to the spell. Compare it to Performance from performance magic and Recitation from Holy Magic. While can do everything Recitation can do and more, and the only thing Performance has on it is that it can sometimes subsume the gestures or vocalizations of spells - which is part of the performance magic virtue, anyway, not the performance duration.
I think the game is better for having these various durations, and I dislike the implication that a duration can be shot down just because something like 'Until', 'While', or 'Not' already covers it.
All that said, I think 'tan' is a silly duration and doesn't feel thematically right for anything in the game. I just disagree with some of the arguments against it.