I responded to your similar query elsewhere, noting that minor and isolated mistakes are usually NO problem.
There is additional information in this query, that your mistake was entering the wrong month. It is not clear to me precisely what the mistake was or how it affects your presence calculation. If you reported an entry date in July when in fact you actually entered on the respective date but in August, that is, you actually entered 31 days later than you report in the presence calculator, that is a fairly substantial error.
How this will affect your application will depend, most of all, on whether you had a margin over the minimum which will accommodate losing 31 days. That is, if your mistake does indeed mean you were in Canada 31 days fewer than you report in your presence calculation, and deducting that 31 days drops your total presence to less than 1095 days, that will be a BIG problem.
Beyond that, if you had a margin over the minimum large enough that this does not mean you are ineligible, how it will affect your application otherwise depends on many other factors and circumstances, on how strong your case is otherwise, how credible you are otherwise, and so on. This is probably a mistake big enough to warrant submitting a correction via the webform (after you have AOR of course).