Tax year starts from July and ends at June. any amount intimated to your employer, backed by supporting, can be adjusted against your tax liability upto June of that specific year. any amount that remains unadjusted / unclaimed or is not able to be setoff completely against your tax liability can not be carried forward to next tax year by any employer. That unadjusted amount can then be claimed through your income tax return for that specific year from FBR (practically one has to forget that claim coz getting refund from govt. is a cumbersome task).
some employers and companies do adjust your withholding tax claims in subsequent years but that is a clear violation of income tax rules and FBR can impose fines and penalties on companies if matter is identified in special tax audit of company.