Hi Ravi,
You mention Acount Key, so I assume you are billing in SD?
Since their are no answers, i can share what we found re IS-U Billing, on the off chance it helps given Tax is pretty central.
We did a tax change back in 2010 and found the Tax Configuration timeslice only relates to the ISU billing document creation date [so an effective from date rather than a proration date].
One scenario did require proration which was achieved using ABAP in Enhancement spot EBIA0002. We then used ABAP in Enhancement spot EBIA0001 to determine the correct Tax code for each Time slice.
For this to work we needed to have different Tax Codes mapped to our Tax Determination Values in TE011 for the old and new Time Slices.
Hope this helps you find a solution.
Best wishes
Neill