The masculine nouns "sat" and "m(j)esec" have irregular plural forms and, unfortunately, have to be memorized; the good news is that words like these aren't too common. The genitive plural of the word "sekund(a)" can end with either "i" or "a" (at least in Croatian). I'll defer to the native speakers on which form is more common!

Thank you for your answer. I learnt cases and that is why I have this question. After the number "pet" it is genetiv plural, and genetiv plural for a masculin noun like "mesec" sould be ending in "-a". So my question is more like : why the genetiv plural of mesec is meseci and not meseca ? Same question for sekund and sat. Are they just irregular or is there another reason ?

Well, it is a difficult question, but I'll try to answer it. First of all, do you the cases? Anyway, the first case in Serbian language is nominative and if you want to say a month it is mesec and months are meseci, but when you put numbers (2 or more) in front of it, the case changes and it becomes genitive. Sometimes the word doesn't change in spelling, but it changes in pronunciation. So, it would be: jedan mesec, dva meseca, tri meseca, četiri meseca, but pet meseci, šest meseci, sedam meseci ... then jedan sat, dva sata, tri sata, četiri sata, but pet sati, šest sati... and jedna sekunda, dve sekunde, tri sekunde, četiri sekunde, but pet sekundi, šest sekundi and so on. Anyway, you need to know the gender of the noun and the case. I don't know if this helpes but I tried :-)