We have the same thing in Russian, for the word sutki, which means 24-hour period (a full day). The singular form is odni sutki, seemingly plural, similar to Polish jedne drzwi, there's no such thing as odna sutka.
Russian "door" dwierʲ is singular and dwieri plural "doors," that's why I asked.
I first wrote "My grandfather..." and it didn't accept. I thought when there are no specifications the family members would be from the person who speaks. The correct was "the grandfather..." but I don't think in english we should put the article in that case (just a feeling, as english is not my language). It is funny how polish people, when speaking other languages, put the articles all the time! Because they don't have it, so it is not natural. As for me, I'm crazy with the declensions!!! I may sound really crazy! Because in portuguese we don't have them either.