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. For reasons I don't want/need to go into I can't simply issue a query to oracle to get the new date.
Does anyone know of a simple and reliable way of adding X number of months to a time_t?
Some examples of the types of calculations are shown below.
30/01/2009 + 1 month = 28/02/2009
31/01/2009 + 1 month = 28/02/2009
27/02/2009 + 1 month = 27/03/2009
28/02/2009 + 1 month = 31/03/2009
31/01/2009 + 50 months = 31/03/2013
Method AddMonths_OracleStyle does what you need.
Perhaps you would want to replace IsLeapYear and GetDaysInMonth to some librarian methods.
#include <ctime>
#include <assert.h>
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);
}
You can use Boost.GregorianDate for this.
More specifically, determine the month by adding the correct date_duration
, and then use end_of_month_day()
from the date algorithms
Convert time_t
to struct tm
, add X to month, add months > 12 to years, convert back. tm.tm_mon is an int, adding 32000+ months shouldn't be a problem.
[edit] You might find that matching Oracle is tricky once you get to the harder cases, like adding 12 months to 29/02/2008. Both 01/03/2009 and 28/02/2008 are reasonable.
Really new answer to a really old question!
Using this free and open source library, and a C++14 compiler (such as clang) I can now write this:
#include "date.h"
constexpr
date::year_month_day
add(date::year_month_day ymd, date::months m) noexcept
{
using namespace date;
auto was_last = ymd == ymd.year()/ymd.month()/last;
ymd = ymd + m;
if (!ymd.ok() || was_last)
ymd = ymd.year()/ymd.month()/last;
return ymd;
}
int
main()
{
using namespace date;
static_assert(add(30_d/01/2009, months{ 1}) == 28_d/02/2009, "");
static_assert(add(31_d/01/2009, months{ 1}) == 28_d/02/2009, "");
static_assert(add(27_d/02/2009, months{ 1}) == 27_d/03/2009, "");
static_assert(add(28_d/02/2009, months{ 1}) == 31_d/03/2009, "");
static_assert(add(31_d/01/2009, months{50}) == 31_d/03/2013, "");
}
And it compiles.
Note the remarkable similarity between the actual code, and the OP's pseudo-code:
30/01/2009 + 1 month = 28/02/2009
31/01/2009 + 1 month = 28/02/2009
27/02/2009 + 1 month = 27/03/2009
28/02/2009 + 1 month = 31/03/2009
31/01/2009 + 50 months = 31/03/2013
Also note that compile-time information in leads to compile-time information out.