Applicability of hike in service tax

Now that Service Tax (ST) has been increased from existing 10% to 12% w.e.f. 01-Apr-2012, I have below mentioned query:

My mobile's billing cycle is from 15th March to 14th April.

I want to know, whether I have to pay 12% for the bill generated for the billing cycle of 15th March to 14th April or will it be 10% for bill between 15th March to 31st March and 12% for bill between 1st April to 14th April.

I understand that in 2nd case, it will be tough situation for mobile companies because of mathematical calculations, however I wish to know what exactly is the rule book.

Kindly provide necessary inputs.