Your current listings would count towards your selling limits, so you don't actually get extra when the month ticks over I'm afraid (otherwise, people could list 100 on one day, and another 100 the next over the last / first days of a month, which would undermine the point of selling limits).
Plus, while I'm not 100% sure because some sellers have reported differently, just from my own experience, on the odd occasion I sell items on this account, my limits are reduced for 30-odd days (that is, the limits weren't applied to me on a calendar-month basis, but a 30-day basis from when the item was listed / sold).
