NPS Contribution query

As we understand employee can claim Tax benefit maximum upto 10% of Basic in a Financial Year. If employee had not taken the benefit of NPS for the 1st 2 months .. can he still claim his NPS benefit for the months he missed and have the same adjusted ? Wanted to check as Employer is saying you can't claim for the missed months ... you can claim only for the current and future months as Instructions are not given.