Personally I never had a problem with the timer, 99% of the time you should be able to finish the mission just in time.
Not all missions are timed. If you do worry about the timer, there is a resistance order that freeze the timer until your squad is revealed (either you use a weapon or detected by enemies). There is also an order that adds extra turns. If you really don't want to deal with the timer, I think there are mods that remove it, but you probably has to ask the author to send you a copy since most of those are on steam.
Post edited January 03, 2021 by zollac