Here's the block:

wait () seconds or until<> ::control

Basically, it does what it says it does: Wait for a bit or until a certain condition is fufilled. I know you can workaround like this. It's a variable timer, but it's not a perfectly accurate solution (so far, there is no scratch script that can recreate a accurate timer as accurate as the built-in timer due to screen refresh):

define Wait (Seconds) seconds or until <Conditon>
repeat until <<Conditon> or <(VarTimer) > ((Seconds) * (30))>>
wait (0) secs
change [VarTimer v] by (1)
end

While there is a warkaround to this block, it will save a lot of time considering the fact that control blocks like these generally are used a lot, and also, the workaround isn't a perfect workaround, and can cause problems in projects where milliseconds matter.

Edit:
I just thought about a more useful block that if added, will replace the above block and add even more functionality. The Repeat () Seconds or until <> block.

repeat () seconds or until <> {

} :: control

This block will repeat a certain script until the time limit is up or the condition is true. And I'm sure someone will ask, “What if the time limit is reached when the loop is still running?” In that case, the loop stops itself from running any more of its code.

About the millisecond thing…

define wait (secs) or until <value>
set [paused time v] to (timer)
wait until << (timer) = ((secs :: custom) + (paused time))> or <value :: custom>>

Semi support. There is a workaround that's simple enough but having it all be in one block would be really convenient
My workaround:

...
set [timer v] to (0)
set [seconds v] to (...)
repeat until <<condition>or<(customtimer)>(seconds)>>
change [customtimer v] by ((1)/(fps))
end
...