30 June 2007
If relating to materials - i.e Excisable Goods, then I do not think that you can claim the same.
If credit being claimed is of Service Tax, if you can prove that the payment was made in the current year, or within the previous one year, then you can claim the Credit of the input.
02 July 2007
As per rule 3 (1) of CENVAT Credit Rules 2004 if payment on input service is made then you are eligible for taking credit.
There is no time limit given for omitted credit to be claimed later. But unless your previous Half Year's ST 3 show closing balance of such amount utilising now and establishing the genuineness to the ST authorities is quite difficult. The logic is simple - since the credit is available on payment basis only one has to strictly go by date of payment only. No genuine payment will generally be skipped for credit taking purpose.