FIDIC claim for delay payment

General contract clause (FIDIC 1987),

Particular Contract Clause 60.10

Time for Payment

The amount due to the contractor under any interim payment certificate issued by the engineer pursuant to this clause, or to any other terms of the contract within 28 days after such interim payment certificate has been delivered to the Employer, or in the case of final certificate referred to in sub clause 60.8, within 56 days after such final payment certificate has been delivered to the Employer. In the event of the failure of the Employer to make payment within the time stated, The Employer shall pay to the contractor compensation at the 28 days rate of KIBOR+2% per annum for local currency and LIBOR+2% for foreign currency, upon all sums unpaid from the date by which the same should have been paid. The provisions of sub-clause are without prejudice to the contractor's entitlement under clause 69.

For Delayed Payments

As it is stated in the contract, KIBOR or  LIBOR+2% per annum, use the annual (yearly values ) of KIBOR or LIBOR for calculation. So get the KIBOR / LIBOR rate for yearly and find a percentage then apply this for the extension period only.
If KIBOR or LIBOR is 10% than +2% = 12% is annual percentage. Than apply 14/12=1, 1% per month.
28 days rate means that the interest charges are compounded monthly (28 days).