The Greek Asylum Service has published an announcement that beneficiaries of international protection, whose residence permits expire before the 31st of March 2019, can apply now to renew their residence permit. The date when it expires is written on the front side of your residence permit, directly under your name. In the picture of the post, the expire date is circled in red. You have to apply to renew your residence permit at least two months before it expires.
This is because it can take two months until a residence permit renewal is processed. So, if you apply for a residence permit renewal too late, then your residence permit might expire before it gets renewed. If you are found with an expired residence permit, this might cause you trouble with the police or other authorities. Here you can find more information about how to renew your residence permit.