Region: EU

I don't think it is necessary and adding a timer script would take resource on memory. Especially when RAM is costly, making a game more resource hungry for an unnecessary funtion would not make sense. You get lot of time to do those Rifts anyway. 

I have an alternative suggestion. I agree that we need to know when dark rifts  spawn. And i agree that we dont need another timer in this game. 

My suggestion: Just make it weekly and and increase the reward proportionally. Spawn them every thursday and thats it.  

5 days is fine as it is now. You can make timers with websites or use phone or calendar on time. 

And it is not a horrible thing if you miss Dark Rift 1 or 2 days since reward is not lowered. Also everytime you log in or swap character the game does highlight that you have Dark Rifts active.

Making it a weekly or thursday reseted, makes players just even more bored as "one more to weekly check list" thing.

Also making it back to 7 days does not require reward increase. Since Dark Rifts were 1st 7-14 day cooldown, then lowered to 5-11 days coold down and now they are at 5 day cooldown.

I must say, I disagree with that opinion. 
1. While calendars are fine, people might not always be able to be online at the same time, making it likely that over time, the time for the next respawn will always be later and later

2. It is not bad to miss the rifts for 1 or 2 days sure. But then the next one is also 1-2 days later, compared to the first spawn. And on and on, meaning that over time, you will get less rewards than someone doing it exactly on time. If the respawn was weekly and players are free to take them on at any time during the week, this would keep the amount of rewards equal for people regardless of their availablity and schedule, which is more fair in my opinion (as someone who has a very errant work schedule)

3. Whether its every 5 days or every week, it is a checklist item anyway, at least to me

4. Comparing the rewards to the past doenst really make sense here, because the baseline we have right now is the 5 days that are intended. While you can argue that the devs set it to 5 days while accounting for players not always doing it on time on average, it still creates a stark discrepancy between players who manage to do it on time and those who dont. It is the same problem of cumulative delay as in the second point. If we however assume that devs intend us to be able to do it every 5 days, then if the respawn time is increased to 7 days by making it fixed to weekly, then it would necessitate an increase in rewards too, by 2/5th using simply mathematical scaling (not accounting for any adjustments for making it easier or more comfortable).

We still need this, please add a cooldown timer of the first to spawn boss if all are on cooldown! Please!

You can now see the spawn times within the new in-game Boss Info window. Thank you for the suggestion 😁

IMO have the spawn at reset 5 days after they were last killed.