What the achievement asks
Complete a single Time Rift. Ten points.
This is the entry ticket to the whole Time Rift branch, and it is the easiest of the ten.
Unlike most of its neighbours, this one is not account-wide. Blizzard ties it to the character, so a second character who wants it has to run its own rift. That matters if you are used to letting one main clear a branch for everyone.
Where and when
Time Rifts open at the Temporal Conflux in Thaldraszus, where Soridormi stands at 51.5 57.1. They start at the top of the hour and run about twenty minutes.
There is no way to start one yourself and no way to join one late enough to matter. If you arrive at twenty-five past, the rift is already closing. Turn up a minute or two before the hour instead.
While a rift is running, its icon appears on the Thaldraszus map. Hovering that icon tells you which invaders are up for this hour, which matters a great deal for two of the other achievements in this branch.
What completing one means
Talk to Soridormi, take the task she offers, do it, report back, and keep going until the rift closes with its final boss.
You do not have to be first, best or fastest. Participation is what counts, and the rift is designed for a crowd rather than for a group, so there is nothing to organise and no invitation to wait for.
Do the rest at the same time
Never run a rift for this achievement alone. The other nine pieces of the branch all happen inside rifts, and several of them depend on which invaders the hour brings. Read the branch first and you will finish four or five of them during the same twenty minutes.
The ones worth knowing about before your first rift are Lock and Load, which only works during an Alliance and Horde invasion, and To All The Squirrels I've BEEN Before, which only works during a Titan invasion. Both are quick when the right hour comes up and impossible when it does not.
Where it fits
One of ten under Closing Time, which rewards the Bestowed Sandskimmer and is a piece of A World Awoken.