Accounting for Unbilled Revenue

This relates to accounting for Unbilled Revenue where transaction takes place in foreign currency (USD).
Scenario goes like this:
ABC Ltd. is a service provider having number of domestic and foreign clients. It does billing to Z Co. Ltd in USD while base currency for ABC continues to be INR.

As per Billing cycle, the bill to Z Co. shall be raised on 15th April 09 for work done for the period Jan'09 to Feb'09.
ABC wants to put entries for Unbilled Revenue on a monthly basis (Jan to Mar).
In this example billing for each month is $1000 (Jan to Mar) and for that of Apr is $500 (till 15th Apr). Also, the currency conversion rate is different at all month ends (as on 31st Jan Rs.45, 28th Feb Rs.47, 31st Mar Rs.46 and 15th Apr (Rs.50). Please help me with the accounting entries for the Unbilled Revenue, complying with the Indian Accounting Standards and business scenario.

Following is what our system is doing right now which seems to be bit dicey:

Accrual  31st Jan  Unbilled AR Dr. $1000  Rs.45000 Conversion Rate - Rs.45
      Revenue     Cr. $1000    Rs.45000

Reversal 1st Feb   Revenue     Dr. $1000    Rs.45000
     Unbilled AR Cr. $1000    Rs.45000

Accrual  28th Feb  Unbilled AR Dr. $2000  Rs.94000 Conversion Rate - Rs.47
      Revenue     Cr. $2000    Rs.94000

Reversal 1st Mar   Revenue     Dr. $2000    Rs.94000
     Unbilled AR Cr. $2000    Rs.94000

Accrual  31st Mar  Unbilled AR Dr. $3000  Rs.1,38,000 Conversion Rate - Rs.46
      Revenue     Cr. $3000    Rs.1,38,000

Reversal 1st Apr   Revenue     Dr. $3000    Rs.1,38,000
     Unbilled AR Cr. $3000    Rs.1,38,000

And Finally, the actual billing happens on 15th April. Nowhere in the above flow of transaction does Currency Exchange Gain/Loss appears which seems to be incorrect. Please guide with appropriate accounting of the above transaction.