Ok this maybe a really stupid question but i can't tell if this game is one were you can explore and rest to your hearts content or if this game should be played as fast as you can.

You see i want to explore but the game has this time system and i can't help but feel like if i spend time killing and looting every place i can i will get some kind of "Bad ending" i'm i right to worry about this or does the whole time thing
not really matter?

PS: is there a cap to the day counter like say it gets to 999 days does it stop or does it go into the thousands?

There is no need to hurry for the main quest, you can (and should) take your time to explore all the areas, rest if needed, travel back and forth as needed, ... An average play-through in my experience, when not hurrying, will take something between 100 and 150 in-game days.

But be careful that some quests have timers. Mostly it's companion quests, many of them when they join you will do it for a reason (hunting bandits or wyverns, free a friend, ...) and this quest usually has a timer. If you don't do it, after a while the companion will start complaining, and if you still don't fulfill the quest, the companion will permanently leave the party.

There are a few other quests with a timer, but only few, and they are usually very explicit about it.

Only certain companions put a time limit on you, plus one event late in the game, which will be made obvious. The companions to watch out for are Jaheira/Khalid and Minsc. For the rest of the game, it's what Baldur's Gate is famous for: explore to your heart's content.

There really isn't much need to rush through the game unless there's a certain companion quest that needs to be done within a certain timeframe. If you plan on keeping them in the party for a little while, I'd probably do what I could to complete that quest if their dialogue seems to indicate they're getting impatient (sometimes it's better to either drop them from your party until you level up just a bit to complete their quest without dying a bunch in the process of completing it). Some companions have little questlines but aren't very strict in the time limit for them, while some others you need to focus on completing that thing for them as soon as you're able.

One thing you may have to rush doing though is getting the companions you want for your party early rather than leveling up a good deal before deciding on what party members you want to carry with you for a majority of the game. Mainly because as some FAQs/guides I've read have pointed out, the HP scaling for NPCs is bad in that if you don't have a good number of the companions join your party while your character is at a lower level, then some companions will have their hit point totals fairly gimped compared to if you had recruited them at a lower level then leveled them up in your party. ITT, they lose out on potential hit points that they would have gotten from leveling up naturally compared to the pre-assigned hit point totals that they get when they scale to your level having them join later. So battling/leveling before getting the compliment of companions you feel comfortable with isn't recommended for reasons such as that one. Missing a potential NPC early at say, level 2 then later finding them in an area when you're level 5 can make a big difference in their hit point progression for some of them (not a great thing if they're a front line combatant). Sadly I think this is just the way the game was coded originally so I don't think there's a way of changing it with a mod.

Unless you are either playing on easy, or you have a tweak/cheat that allows maximum HP at level up, this is not the case. For starters, companions have multiple creature files that scale (at first meeting) with your level. If you meet Kivan at level 1, he will be set at level 1. If you meet him at level 5, he will be set at level 4. If you meet him at level 6, he will be set at level 6. These instances have a set amount of XP. Any XP added will be based upon a dice throw (D10 for Kivan), unless the aforementioned tweak in in play. Bottom line: meeting an NPC later does not necessarily mean he/she will be gimped, since some are not even available until very late game.