Yeah... I thought that maybe through some cosmic weirdness they have a different number of days per month but then I realised that the reason I noticed there was no 31 is because I wanted to look at when the Moon, Mars and Venus singles were coming out (which is March 31st) and looked on my calendar. Then I double checked on the calendar on Hama Chisaki's site and they had a 31 too. So it's an error.