Time Bank is a mechanic added that saves time after a skill is complete. The maximum amount of time saved for non-vip is 24h, the maximum for VIP is 30 days. This allows you to sleep at 3am when a skill will finish and can use that time when you wake up to add to the next skill or pip of the current skill you were training.
If you are training right now in 5.3 and you missed some time, I suggest you dump some of that banked time so you can use it.
Each Group PROFESSION, RACE & CLASS has their own BANK so empty them all!