Return to Answer

Don't do this, compare for example Why is "using namespace std" in C++ considered bad practice?. And actually your code does not rely on it, you can simply remove that line.

As already mentioned in the comments, neither the argc/argv parameters nor the return statement is required in C++ (see e.g. What should main() return in C and C++? for an overview):

Don't do this, compare for example Why is "using namespace std" in C++ considered bad practice?. And actually your code does not rely on it, you can simply remove that line.

As already mentioned in the comments, neither the argc/argv parameters nor the return statement is required in C++ (see e.g. What should main() return in C and C++? for an overview):

void askForBirthdate(int &dayOfBirth, int &monthOfBirth, int &yearOfBirth){
 // ...
}
int main()
{
 int dayOfBirth, monthOfBirth, yearOfBirth;
 askForBirthdate(dayOfBirth, monthOfBirth, yearOfBirth);
 int ageInSeconds = calcAgeInSeconds(dayOfBirth, monthOfBirth, yearOfBirth);
 std::cout << "You are " << ageInSeconds << "old.\n";
}

Converting day/month/year to a time value (according to the local timezone) is easily done with mktime():

time_t timeForDate(int day, int month, int year){
 struct tm timeinfo = { 0 };
 timeinfo.tm_mday = day;
 timeinfo.tm_mon = month - 1;
 timeinfo.tm_year = year - 1900;
 timeinfo.tm_isdst = -1;
 return mktime(&timeinfo);
}

void askForBirthdate(int &dayOfBirth, int &monthOfBirth, int &yearOfBirth){
 // ...
}
int main()
{
 int dayOfBirth, monthOfBirth, yearOfBirth;
 askForBirthdate(dayOfBirth, monthOfBirth, yearOfBirth);
 int ageInSeconds = calcAgeInSeconds(dayOfBirth, monthOfBirth, yearOfBirth);
 std::cout << "You are " << ageInSeconds << "old.\n";
}

Converting day/month/year to a time value (according to the local timezone) is easily done with mktime() (the "counterpart" to localtime()):

time_t timeForDate(int day, int month, int year){
 struct tm timeinfo = { 0 };
 timeinfo.tm_mday = day;
 timeinfo.tm_mon = month - 1;
 timeinfo.tm_year = year - 1900;
 timeinfo.tm_isdst = -1;
 return mktime(&timeinfo);
}

Some other miscelleaneous remarks:

• You can get rid of the function declarations if you define all functions before using them.

• Statements like

   int ageInSeconds=0;
   ageInSeconds=calcAgeInSeconds(monthBorn,dayOfMonthBorn,yearBorn);
  

can be combined to

 int ageInSeconds = calcAgeInSeconds(monthBorn, dayOfMonthBorn, yearBorn);

And use more (horizontal) space!

• Check and fix all compiler warnings. For example, in calcOffset(), curYear and three more variables are computed but their values are never used.

• Always use curly braces { } with if-statements, even if the if or else part consists only of a single statement. That helps to avoid errors if the code is edited later.

• The long if/else if/else if/... statement in calcDaysRemainingThisYear() can be simplified by using a switch statement.