Master of the Turbulent Timeways

If you enter the next week and complete a TW dungeon on a character that still has the Mastery of Timeways buff active from the last week, you will gain credit for another week towards this achievement by just completing that single dungeon.

It's very simple. Every week during the Turbulent Timeways event you will have to do 4 timewalking dungeons that each give you a stack of Distilled Knowledge of Timeways that lasts 2 hours.
Once you reach 4 stacks it turns into Mastery of Timeways. Do keep in mind that the Distilled Knowledge of Timeways buff is timed so you will have to do the next dungeon while you still have this buff. A new stack resets the timer.
Do this for all 5 weeks (until November 1st) and bam you have the achievement and the Sandy Shalewing mount

To add what is being told in comments as well, this achievement is account-wide so you don't have to use the same character every week.

So, if I didn't get to do week 1, this achievement becomes obsolete?