How to Create new Periods in Assets Calendar

Hi,

We have done all the setups related to the Assets module and as per our client's Request we want to add new period type to the Assets calendar.

We have defined our Assets calendar from 01-APR-87 and we are using daily prorate calendar.

But we have defined our Assets calendar from 01-Apr-13 and now system is not allowing us to add assets in Before APR-13. 

Please let us know how can we add new periods to the Defined Assets Calendar.

Thanks ,