China Unicom 3G Sim Card recharge and packages

Hello,

I am confused, who the china unicom count the monthly period for the packages. In Europe, for prepaid card, the package starts at the day you booked it, and is over after 30days, so if i book a package in 13th of the month, i can use this till 12 of the next month. But the china unicom store told me, the just can book the packages in the 1st of the month and its over in the last day of the month. it's not possible to get a package between this time.
is this correct? and how can i book a package? just recharge? or some more actions?
the shop doesn't know excatly, and the hotline also don't understand what i want to do.
now my two sim cards, are empty, because the package or the month is over, and i just can use the call by call rate (0.1RMB/MB)

thank you very much