Standard calculation problems. Go waaaay back to where I said you have to decide if you are rounding during output or during calculations. These are things that are based on business rules.
Not sure exactly what step you ended up with 92 though, there are only 91 days between them (1 + 31 + 30 + 29 = 91). As timestamps when you divide them though, you do end up with a fractional value (which is normal during calculations). So your problem is precision loss since the same will not necessarily apply to all values, so you cannot reliably choose round, floor or ceil as your options without reversing it to verify.
Make use of the DateTime objects and DateInterval instead. This will give you an object that has the number of days that have passed and is more reliable and locale aware. It will require PHP 5.3+ to operate.
header('HTTP/1.1 420 Enhance Your Calm');