My part is level 17 and one of my PC’s is a sorcerer. He discovered a combo using the extended spell with time stop to just freeze time. Time stop lasts a minimum of 2 rounds or 12 seconds so by using 13 sorcery points he could freeze time for 24h. He could then use 8 of those 24 hours to take a long rest regaining his points and 9th level spell slot. He could then do whatever he wanted and just before time unfreezes he could recast it, rinse and repeat over and over again . Unless he affects another creature he can keep time frozen as long as he chooses. I’m not so worried about the OP of this because I can introduce scenarios and enemies specifically tailored to counter this ability (homebrew if you were wondering) but I do wonder if this Freezing time strat is viable by official and adventurers league rules?