Add two dates in C++

Question

I'm sure this question is answered elsewhere, but I cannot find it on Google or SO, so here goes.

In C/C++, I want to convert a relative time in format dd-hh:mm:ss provided by

ps -o etime

to an absolute UTC formatted date.

This doesn't seem like it should be very hard. Supposing I have already got a function to produce the relative time stored in struct tm format:

struct tm *starting_rel_time = my_reltime_converstion(...);
time_t t = time(0);
struct tm *current_abs_time =  localtime(&t);

what I want is basically the opposite of difftime:

struct *tm starting_abs_time = current_abs_time - starting_rel_time;

Now, I can write my own function to do the conversion, but it's a nightmare because of all the carry operations and special conditions (leap years etc.). Surely there is a way to do this in the C/C++ libraries?

Answer 1

Convert the DD-HH:MM::SS to seconds with simple math; it's relative-time, so just multiply and add. Then, query the current time() in seconds (assuming it's "relative to now"), and add them. Then use gmtime to convert back to a struct tm.

Answer 2

Use Boost::Date_Time libraries.

Answer 3

There is no such language as C/C++.

If you're asking about C, I suggest representing dates internally with a simple numeric type, and converting to and from struct tm only when necessary. If you only need to cover a few decades, then you could use time_t and convert using the standard gmtime and mktime library functions. To cover a wider timespan, you could use a Julian day representation.

If you're asking about C++, I suggest the Boost.Date_Time library. Of course, the C library functions are still available if they meet your needs.

Answer 4

What you're trying to do doesn't make sense. You cannot add two dates. (And difftime doesn't return a date, nor a time_t.)

In practice, on most, if not all implementations, time_t will be an integral type with the number of seconds since some specific "epoch". On such machines, you can add or subtract an integral number of seconds from a time_t to get a new time, at least if all of the times you're interested in are in the interval supported by time_t (roughly between 1970 and 2038 on most Unix platforms). This, along with gmtime, mktime and localtime is probably sufficient for your needs. Note especially that mktime is required to "correct" it's tm input: you can, for example, take a tm, add 5 to the field tm_mday, call mktime on it, and get the correct values for a date five days in the future—all of the necessary carry operations and special conditions are handled in mktime.

If this is not sufficient, C++11 has both a time_point and a duration class, with (from a quick glance) seems to have all of the functionality you could possibly need.