Overloading the '-' operator

Overloading the '-' operator

Currently I'm writing a program that has a section to determine the
difference in days between two dates, but by overloading the minus
operator.
I'm currently staring at my screen drawing a complete blank. I have some
fleeting thoughts in my head but they are exactly that, fleeting.
What's to happen in the main.cpp is that there are going to be two
variables, for instance beethovenDeathDate and beethovenBirthDate that
will be subtracted to determine how long he lived for. Which is something
around 22000 days if I recall correctly.
So without further ado, here is my code:
Date.cpp
const std::string Date::MONTH_STRINGS[] =
{
"", //one based indexing
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December"
};
const int Date::DAYS_PER_MONTH[] =
{
0, //one based indexing
31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};
Date::Date(int day, int month, int year) : _year(year), _month(month),
_day(day)
{
isValid();
}
Date::Date()
{
time_t t = time(0); // get time now
struct tm * now = localtime( & t );
_year = now -> tm_year + 1900;
_month = now -> tm_mon + 1;
_day = now -> tm_mday;
}
int Date::maxDay(int month, int year)
{
int ret = DAYS_PER_MONTH[month];
if(isLeapYear(year) == true && month == 2)
{
++ret;
}
return ret;
}
void Date::addDay(bool forward)
{
if(forward)
{
if(_day < maxDay(_month, _year))
{
++_day;
}
else
{
_day = MIN_DAY;
++_month;
if(_month > MAX_MONTH)
{
_month = MIN_MONTH;
++_year;
}
}
}
else
{
if(_day <= MIN_DAY)
{
--_month;
if(_month < MIN_MONTH)
{
_month = MAX_MONTH;
--_year;
}
_day = maxDay(_month, _year);
}
else
{
--_day;
}
}
}
std::string Date::toString() const
{
if(isValid() == false)
{
return std::string();
}
std::stringstream ss;
ss << MONTH_STRINGS[_month] << " " << _day << ", " << _year;
return ss.str();
}
bool Date::isValid() const
{
if(_month < MIN_MONTH || _month > MAX_MONTH)
{
std::cerr << "Invalid date " << std::endl;
return false;
}
int daysThisMonth = maxDay(_month, _year);
if(_day < MIN_DAY || _day > daysThisMonth)
{
std::cerr << "Invalid date " << std::endl;
return false;
}
return true;
}
bool Date::isLeapYear(int year)
{
if(!(year % 4))
{
if(!(year % 100))
{
if(!(year % 400))
{
return true;
}
else
{
return false;
}
}
else
{
return true;
}
}
else
{
return false;
}
}
bool Date::isLeapYear() const
{
return isLeapYear(_year);
}
bool Date::isLeapDay() const
{
return isLeapDay(_day, _month, _year);
}
bool Date::isLeapDay(int day, int month, int year)
{
if(day == 29 && month == 2 && isLeapYear(year) == true)
{
return true;
}
else
{
return false;
}
}
void Date::addYears(int years)
{
if(years == 0)
{
return;
}
if(isLeapDay() && !isLeapDay(_day, _month, _year + years))
{
_day = Date::DAYS_PER_MONTH[_month];
}
_year += years;
}
void Date::addMonths(int months)
{
if(months == 0)
{
return;
}
int deltayears = months / MAX_MONTH;
int deltamonths = months % MAX_MONTH;
int newMonth = 0;
if(months > 0)
{
newMonth = (_month + deltamonths) % MAX_MONTH;
if((_month + deltamonths) > MAX_MONTH)
{
++deltayears;
}
}
else
{
if((_month + deltamonths) < MIN_MONTH)
{
--deltayears;
newMonth = _month + deltamonths + MAX_MONTH;
}
else
{
newMonth = _month + deltamonths;
}
}
if(_day > maxDay(newMonth, _year + deltayears))
{
_day = maxDay(newMonth, _year + deltayears);
}
_year += deltayears;
_month = newMonth;
}
void Date::addDays(int days)
{
if(days == 0)
{
return;
}
if(days < 0)
{
for(int i = 0; i > days; --i)
{
addDay(false);
}
return;
}
for(int i = 0; i < days; ++i)
{
addDay(true);
}
}
std::ostream& operator<<(std::ostream& os, const Date& date)
{
os << date.toString();
return os;
}
Date Date::operator+(int days) const
{
Date ret = *this;
ret.addDays(days);
return ret;
}
Date& Date::operator+=(int days)
{
addDays(days);
return *this;
}
//This is where I get stumped (the parameters was just one of my failed
experiments
Date& Date::operator-(int day, int month, int year)
{
}