Baldur's Gate follows a Time System based on the AD&D system. Days in the world of Toril last 24 hours, just like in the real world.
Like in the real world, there are 60 game seconds in one game minute, 60 game minutes in one game hour, and 24 game hours in one game day. There are 12 game months in a game year as well. However, one game week is called a tenday and lasts 10 game days, and one game month always has 30 game days.
Another difference is the rate at which time passes in the game, which is twelve times faster than real time. That means conversion from real time to game time happens at a factor 12: One game hour, or 60 game minutes, equal 5 real minutes. One game day, or 24 game hours, equal 2 real hours.
The time system during combat
Combat is divided in rounds and turns. One round equals 6 real seconds, and one turn equals 10 rounds or 1 real minute.
1 round = 6 real seconds = 72 game seconds 1 turn = 1 real minute = 12 game minutes 1 game hour = 5 real minutes = 5 turns 8 game hours = 40 real minutes = 40 turns 1 game day = 2 real hours = 120 turns
A spell that lasts for 2 turns and 3 rounds, like Mirror Image at level 20+, will stay active for 2mn18s of real time, or 27mn36s of game time.
A spell that lasts for 2 game hours, like Spirit Armor, will stay active for 10 real minutes, that is to say 10 turns of combat (100 rounds).
What you can do in one round
The number of actions a character can accomplish in one round, i.e, in six seconds of real time, is limited. Some actions can only be performed once per round. These actions are: casting a spell, using a quick item such as a potion or a scroll, using an ability from an equipped item such as the Cloak of the Sewers' polymorph, or using an innate ability like the Kensai's Kai or a Spell Trigger. In any given round, it is only possible to use one action taken from all categories combined. It should be noted that the wizard spell Improved Alacrity lifts this restriction for spells and innate abilities, which can then be used at will.
Without Improved Alacrity, if a character wants to perform two of these actions in succession, a full round must pass between the moment he starts the first one and the moment he starts the second one. Weapon or fist attacks are not subject to this limitation, though. Often, combat involves creatures or characters able to attack more than once in a single round. This can be due to multiple attack forms (like claws and bite), skill with a weapon, or character level.
A character can therefore initiate combat by throwing a quick projectile like the Boomerang Dagger, then cast a quick spell, then resume attacking until the end of the round, assuming they have more than one attack per round. But they cannot use a potion, the Cloak of the Sewer or the ability Kai, because they already used a spell, which belongs to the one-action-per-round group.
The casting time of a spell is the number of tenths of rounds that it takes to cast the spell. A tenth of a round is 0.6 seconds of real time. Casting times range from 1 to 9, therefore, the quickest spells are cast in 0.6 seconds, while the slowest spells are cast in 5.4 seconds.
A couple exceptional items reduce casting time, like the Robe of Vecna and the Amulet of Power. Even with those items, the minimum casting time for a spell is 0, which means it is cast instantaneously.
Rate of Fire
Bows, crossbow, and many other missile weapons have different rates of fire (ROF) —the number of missiles they can shoot in a single round. Arrows can be shot and daggers thrown at a rate of up to two shots per round. Some weapons, like crossbows and slings, take a long time to load; while others, like throwing axes, are too large to throw quickly, and can thus be fired only once per round. Darts can be hurled at a rate of three per round.