How *exactly* is Leave to Enter calculated?

Ok I don't want to lose the good graces of this amazing forum/community, but I need this question answered as try as I might to pin down an exact answer, the internet gives me nothing (incl. UK's .Gov immigration pages)

When I was stamped 6 months leave to Enter as an American, on the 12th of Feb, I already had the same 6 month stamp given the first time I entered the UK a month before (I had to pop over to see family in gernany). I was questioned thoroughly because I was with my now-fiancee, but got a second 6 month stamp.

I feel confident that the 6 months is counted from whichever stamp is most recent. In fact that my leave to enter is always counted from whatever the most recent stamp is, even if I had a shorter length stamp that came within the period of a previous stamp.

If I'm wrong, please correct me!

My real question though is this:

The 6 months is counted to the day, not the month? So that in my case, I must leave by the 12th of August, as I was stamped in for 6 months from 12 Feb.

I just want to make sure it isnt the case that it's by month alone, e.g., I could have entered Feb 25, but I still have to leave Aug 1.

I read that there is some exception for the final days of each month, so that if for example I was to enter on the 31st of August, I would have to leave on the last day of February, and not by March 3 (this seems obvious, and to point to it being calculated by both date and month, and not month alone)

I'm sure I'm not the only one who has sweated this detail. I didn't see any posts answering this when I searched this forum, pardon if this very question has been answered explicitly before.

Hopefully someone wondering this same thing will find this post via Google one day

I feel confident that the 6 months is counted from whichever stamp is most recent. In fact that my leave to enter is always counted from whatever the most recent stamp is, even if I had a shorter length stamp that came within the period of a previous stamp.

If I'm wrong, please correct me!

No, you're correct.

When you left the UK the first time, your first 6-month stamp became invalid and was replaced by the new 6-month stamp.

My real question though is this:

The 6 months is counted to the day, not the month? So that in my case, I must leave by the 12th of August, as I was stamped in for 6 months from 12 Feb.

Yes, that's correct... though I would make sure you leave at least a couple of days before August 12th, just in case.

I read that there is some exception for the final days of each month, so that if for example I was to enter on the 31st of August, I would have to leave on the last day of February, and not by March 3 (this seems obvious, and to point to it being calculated by both date and month, and not month alone)

There are no 'exceptions' -  you get 6 months exactly. So if you entered August 31st, you would have to leave by Feb 28th. Not leaving until March 3rd would make you an illegal overstayer.

I'm not sure if you are, but as you mention the end of August, just for your information:
If you were thinking of coming back to the UK again soon, while there are no set rules for time spent in the UK, if it looks like you are using a visitor visa to live in the UK (i.e. you have spent more time in the UK than in the US in the last few months), you can be refused entry when trying to come back in again. And if you are refused entry, that will make any future visa applications more complicated and delay the processing time as they have to look into the reasons for the refusal.

The general rule of thumb is to spend at least as much time outside the UK as you did inside the UK... so if you have just spent 6 months here, you should go back to the US for at least 6 months before attempting to return on a visitor visa (this doesn't count if you are applying for a fiance or spousal visa though, just for visitor visas).

Indeed, I dont plan to return to the UK until I am approved for a fiance Visa (or spouse, if we manage to marry in the US while my fiancee is visiting). I can handle a little time away if the next time I enter is on a proper Visa

I currently have my homeward flight booked for 4 days before my stamp runs out, and I think I could conservatively expect to be granted a visa by late November.

 I think we'd send off the Visa application in early October, and I'd pay for an expedited review.

Oh, and I misused the word exception. Completely backwards, in fact! I just meant to highlight the case that for entry on a date, that the month 6 months later does not have (for the month, 6 months down the road has fewer days) one has to leave by the last day of that month, and cant include days from the following month, on the grounds that the month of entry had more days.

Well.. enough from me. Thanks for your knowledge and time.