Problem with Date Operations - End of Prior Month

I am trying to dynamically set the end of the prior month so I can use this date as a filter. This is the formula that I am using: DateTimeAdd(DateTimeLastOfMonth(),-1,"months"). When I run this now (on 6/29/17), this is the result: 2017-05-30 23:59:59. Why isn't it giving me 5/31 rather than 5/30? I'm not sure if this is a bug or if there is something wrong in the formula.