How long do I have to claim an input tax credit I forgot to include on an earlier HST return?

A GST/HST registrant generally has a set number of years from the end of the reporting period in which an input tax credit could first have been claimed to actually claim it on a later return, rather than being limited strictly to the specific period the underlying purchase was made in. This is what lets a business go back and pick up a legitimately missed input tax credit on a current return instead of having to amend the original filing from years earlier.

That claim window isn't unlimited, though, and it can be shorter for certain categories of registrants than for others, generally, larger businesses and some other specified categories of registrant face a shorter time limit than most small and medium-sized businesses do. Because the applicable time limit depends on what kind of registrant you are, and the exact current limits shouldn't be assumed from general knowledge, check your specific claim period with your accountant before assuming an older missed credit is still available.

If you do find an eligible missed input tax credit within the applicable window, it's generally straightforward to include it on a current return rather than refiling the original one, but confirm both the eligibility and the timing with your advisor before doing so.