I know that both can mean "inside" but what I don't have clear is whether both mean the same when talking about time. For example:
According to this link http://oilpatchwriting.wordpress.com/2012/02/03/in-vs-within/ within means a limit that can be exceeded, so I'm wondering too if say "the party is within two days max" is correct or redundant.
"The party is in two days" means the party will take place in two days time, i.e. if it's Thursday today, the party will take place on Saturday.
"The party is within two days" means either 'the party will take place some time between now and two days from now', or, perhaps more commonly, '[we] are within two days of the party', i.e. the party will be on Saturday as above. If the former of these two definitions is intended then "the party is within two days" has the same meaning as "the party is in two days maximum", if its the latter definition then both "the party is within two days" and "the party is in two days" refer to a party taking place in two days time and not before.
"Within" does not imply "a limit that can be exceeded", but the contrary.
The 'max' in "the party is within two days max" is redundant as you seem to suspect.
Within is regarded as specifying an upper limit: within two days means maybe today, maybe tomorrow, but no later than two days from now. In is sometimes regarded as more precise: in two days could mean two days from now – particularly with a scheduled event, such as a party.
That said, there are exceptions where the word in can be more ambiguous. If I tell my supervisor on a Monday:
Boss, I'll have that report ready in two days.
that could mean it'll be on his desk on Wednesday, or it could mean it'll be on his desk before Wednesday.
That being the case, it might be preferable to use on or before when you want to reduce the risk of ambiguity.
This is another calendric/non-calendric time unit distinction. Fillmore discusses this in his Deixis Lectures:
"There are a great many devices for indicating the relative times of two events -- that is, devices for identifying the time of one event relative to the time of another event. Some of these have to do with time units of the sort we have been discussing. Thus, if I say She divorced Schwartz and married Harry in the same week, I have indicated two events as occurring within a single calendar week; but if I say She divorced Schwartz and married Harry within a week, I have located the two events as having occurred within a single seven-day stretch, but this time it need not be coterminous with the calendar week.
"Similarly, if I say that one thing happened a week later than another, I say that there is a seven-day-long span between the two events; if I say the one thing happened the next week after the other, I say that the two events happened in two successive calendar weeks, but I haven't said whether the time between the two events is three or six or ten days."
