If you have manual books of accounts:

As per the BIR:
The registration of a new set of manual books of accounts shall only be at the time when the pages of the previously registered books have all been already exhausted, provided, that the portions pertaining to a particular year should be properly labeled or marked by taxpayer. This means that it is not necessary for a taxpayer to register/stamp a new set of manual books of accounts each and every year.

In other words, you only have to get new books stamped if you've exhausted the previous one. :)

If you have looseleaf books of accounts:

You need to bind your looseleaf books for the year and have them stamped by the BIR 15 days after the calendar year ends. So technically, it's January 15.

You'll also have to renew your registration to use your looseleaf books of accounts.

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