initially it has deducted 1,000 per month from Jan to Apr, so D1 is 16,000, then on May if i add 20,000 on C4 and make it 1,500 on B5, D1 is only 14,000 it should now be 36,000 and B7 is Jan-16 it should now be Apr-17 --- (1,500 per month May-15 to Apr-17)

another example problem on May if i add 20,000 on C4 and make it 2,500 on B5, D1 should now be 36,000 and B7 should now be Jul-16 --- (2,500 per month May-15 to Jun-16 but 1,000 only for Jul-16) how do i show that it should only be 1,000 for the last month?

i think a vba code is best suited to handle the problem but i dont know how to manipulate dates using vba code