In the source code I found this:
//getPlayerPremiumDays(cid)
lua_register(m_luaState, "getPlayerPremiumDays", LuaInterface::luaGetPlayerPremiumDays);
int32_t LuaInterface::luaGetPlayerPremiumDays(lua_State* L)
{
return internalGetPlayerInfo(L, PlayerInfoPremiumDays);
}