Around 1752 we changed from the Julian calendar, which had 10 months (hence the dec (meaning ten) in December), to the Gregorian calendar, which has 12 months. So when analyzing your Quaker dates, it is important to consider which calendar was used in the recording of events. If you have the date written as "17xx, 4, 30", then it is important to consider whether or not the record means April 30, 17xx, or June 30, 17xx.
For more facts about the names of the months, I found this site interesting:
And for an easy reminder of the shift in dates (especially those pertaining to Quaker records), there is an easy chart here: http://ftp.rootsweb.ancestry.com/pub/roots-l/genealog/genealog.quakerc1
Happy tree climbing!