Sandy Shalewing

You must get [spell=423861] buff once a week for 5 total weeks to get Master of the Turbulent Timeways. To get the achievement in 1 event cycle, you'll need to get [spell=423861] each week during the event since the event is only 5 weeks long. If you miss a week, your progress is saved, but you will need to wait until the next Turbulent Timeways event to finish the remaining weeks to complete the achievement.

In order to get [spell=423861], you need to get 4 stacks of [spell=423860]. You get [spell=423860] by completing a timewalking dungeon. Note that [spell=423860] has a duration of 2 hours that refreshes every time you get another stack. If it falls off before you get 4 stacks, you have to start over. It's possible to do more than 4 timewalking dungeons during the week and not get [spell=423861] if you let [spell=423860] fall off before obtaining 4 stacks.

The next Turbulent Timeways event is scheduled for patch 11.0.7 with a new mount reward: Timely Buzzbee.

Sandy Shalewing is being added to timewalking vendors according to this post.

edit: added some additional clarity; included info now that a new event has been announced.

To get this mount you need the achievement Master of the Turbulent Timeways. To make it a simple tl;dr you need to do 4 TW dungeons a week to get the required buff. Do this for all 5 weeks that we have TW dungeons and you get the mount.

So will this achievement be finished in one week? if I missed the first week, can I still do this achievement?