easy way to add 1 month to a time_t in C/C++

Question

I have some code that uses the Oracle function add_months to increment a Date by X number of months.

I now need to re-implement the same logic in a C / C++ function.

Answer 1

Method AddMonths_OracleStyle does what you need.

Perhaps you would want to replace IsLeapYear and GetDaysInMonth to some librarian methods.

#include 
#include 

bool IsLeapYear(int year) 
{
    if (year % 4 != 0) return false;
    if (year % 400 == 0) return true;
    if (year % 100 == 0) return false;
    return true;
}

int daysInMonths[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

int GetDaysInMonth(int year, int month)
{
    assert(month >= 0);
    assert(month < 12);

    int days = daysInMonths[month];

    if (month == 1 && IsLeapYear(year)) // February of a leap year
        days += 1;

    return days;
}

tm AddMonths_OracleStyle(const tm &d, int months)
{
    bool isLastDayInMonth = d.tm_mday == GetDaysInMonth(d.tm_year, d.tm_mon);

    int year = d.tm_year + months / 12;
    int month = d.tm_mon + months % 12;

    if (month > 11)
    {
        year += 1;
        month -= 12;
    }

    int day;

    if (isLastDayInMonth)
        day = GetDaysInMonth(year, month); // Last day of month maps to last day of result month
    else
        day = std::min(d.tm_mday, GetDaysInMonth(year, month));

    tm result = tm();

    result.tm_year = year;
    result.tm_mon = month;
    result.tm_mday = day;

    result.tm_hour = d.tm_hour;
    result.tm_min = d.tm_min;
    result.tm_sec = d.tm_sec;

    return result;
}

time_t AddMonths_OracleStyle(const time_t &date, int months)
{
    tm d = tm();

    localtime_s(&d, &date);

    tm result = AddMonths_OracleStyle(d, months);

    return mktime(&result);
}