The security cam shows 10:30pm on 10/4. Why do I know it's 10/4? I can't explain. Look for the chronology I put up, it's in my signature.
Or let me try, briefly:
I come out and say it is fact that we have seen 4 days (starting on 10/1), including episode 4, in NYC (currently we are already on 10/5). Just follow the events and they always wrap up to the next day. (If they don't, it will make Niki's timeline an even bigger mess, but let's forget about that for the time being.)
That means, it's 10/4 right then, when Nathan is on his way to Vegas.
Peter is going over to Mohinder that day, then he's on the subway and in this episode we saw him at Isaac's, which must be 10/5. Why?
Because of Hiro and Ando. The phone calls Hiro makes connect Tokyo with NYC, so I can safely say it's 10/4 in Tokyo when they go to Vegas (because Isaac received the call on 10/3 in NYC) and they will end up in 10/4 in Vegas, because of time zones.
Now, it doesn't matter how long they could stay at the Montecito without anything to tell me what time it is. Because we have Peter and he gets a call by Hiro in this episode. Hiro is 3 hours earlier than Peter. That means, since Peter is on 10/5 (I hope you can agree with me on this), Hiro can't be on 10/6 or something. It must be 10/5 in Vegas.
And that means, Hiro and Nathan meeting at the diner was the morning of 10/5.
If you try to follow this argument, I can finish this up. (I had a hard time getting it right, because this stuff, believe it or not...even though I actually posted a timeline, confuses the hell out of me. Once I get to one point, I forget about the other. That's why I need it written down or I will simply not get heads nor tails of anything, any more.)
So, Nathan and Hiro is 10/5. I pass it on as fact. Nathan arrived 10/5 at the Montecito. Now tell me...is there 10:30pm between 12:22am (Nathan's arrival) and morning/noon of 10/5 (Nathan and Hiro talk), when he could have had sex with Niki? That's the other half of the day.

Which means, the 12:22am on the bill is either not Vegas time or this is wrong (if everything I pieced together so far is right...if not, we can just finish talking about this, of course).
Two things you have to know:
1. I agree that Nathan might have stayed at the Montecito for a day and all this is ok. But he didn't. I just tried to explain, why.
2. My assumption: He leaves 10/6. When he signs the bill he's not leaving. It is not 10/6. It is 10/5 and he will leave tomorrow, in terms of the show. If this assumption is wrong, no biggie, then it's 10/6 for Nathan at the end of this episode...doesn't change my point.
Just to comment on that last part, but this is actually not important for the topic at hand:
Mixing up two stories in the same time zone (Niki and Nathan) is not good style, but I can't assume that they won't do it (assuming won't get you anywhere with Niki's timeline, anyway) and I consider it possible. Which means, it could be 10/6 when Nathan signs the bill and he's leaving right then.
But, as I said, the topic is his arrival and not his departure.

Freelancer:
I have watched 5 episodes of this show. In every single episode they put some kind of paper or book with writing in the shots. They really shouldn't make any mistakes with those. But I have to assume they did, you are right. Or else...uh-oh.
Or else I will give up on timelines, right now. Promised. I don't need them.
LOL
Edit: Sorry, I got confused about those damn dates there for a sec...no, it doesn't destroy my whole argument, it was just a mix-up right on the top. Now it's ok.