Should I go to the mission or hunt UFO?

I was playing Long War. I was suggested a covert operative extraction mission which is going to expire in 48 hours. Just as I was going to send a squad to the mission a new UFO appeared (small, Fighter, flying high) ... Should I hunt the UFO now to go for the operative first?

I know that hunting the UFO could take more than one interceptor to send for it and would take time. I worry the operative extraction mission could expire while I am hunting.

On the other hand transporting the squad to the extraction place and back takes a few hours. By the way, what is the usual range of transportation time?

I do not know if the game's time continues to run during the mission itself, does it?
If it does run; is it a fixed amount of time per battle; fixed amount per each turn of the battle; or some other formula?

If it does not, I probably should go to the mission first; because after completing the mission I would still have time to hunt the UFO before it disappears. By the way; what is usual time of UFO staying around before disappearing?
What is the usual time of one interception attempt?

Time stops during missions.

I can't find any citation indicating that time stops during a mission, but from my memory of playing through the game several times I feel pretty confident saying that it does.

This would be easy enough to check. Save, start a mission, see what time it is when you get there, play some turns, leave, check the time.

But not when flying to the mission site.

But in this version of X-Com, you don't have much control over your aircraft. The skyranger will fly to the site, taking up time, and fly back when it's done, without you being able to stop it and send your interceptor.

So check.

This means that maybe you can do the mission first, and maybe not. To be sure, save the game, head to the mission site, leave immediately, and return to base. If you can still intercept the UFO, then you're fine doing the mission. Load, and actually do it.