How to calculate time differences in C++ with time_t before the epoch?

Question

What I would like to do with my simple program is to calculate a difference in seconds between two dates.

time_t referenceDate;
time_t dateNow = time(0);
struct tm referenceDateComponent = {0};
referenceDateComponent.tm_hour = 0;
referenceDateComponent.tm_min = 0;
referenceDateComponent.tm_sec = 0;
referenceDateComponent.tm_year = 89;
referenceDateComponent.tm_mon = 11;
referenceDateComponent.tm_mday = 31;
referenceDate = mktime(&referenceDateComponent);  
long seconds = difftime(dateNow, referenceDate);

Whit the code above the application works fine, but if try to set tm.year negative (to build a date before 1900) the mktime() function return -1

I know that time_t type manage only dates starting from Jan 1, 1970 UTC according with the documentation:

For historical reasons, it is generally implemented as an integral value representing the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC (i.e., a unix timestamp). Although libraries may implement this type using alternative time representations.

I know there are also the Boost libraries but is not a usable solution for me.

So my question would be, there is any way to get difference in seconds from dates starting before 1970?

Answer 1

I recommend using the C++11 std::chrono namespace and <chrono> standard headers and the standard functions and classes inside them.

You might also consider difftime from the C standard and localtime & mktime

And there are a lot of good other reasons to upgrade to C++11 at least (or C++14 if you can). Several good recent free software compilers GCC and Clang/LLVM support that standard (compile with -std=c++11 or -std=gnu++14 if you want GNU extensions & C++14)

BTW, your question is much more complex than you believe. Calendars has changed. Julian/Gregorian calendar transition happened in the XX^th century in Russia. My late mother was born in 1919, during emigration and the civil war, in a Russian village whose legal system was disputed at that time (Russian revolution did not occur instantly). She had some papers mentioning 13^th december 1919, and other papers mentioning 26^th december 1919, referring to the same date in two different calendars. How would your software deal with that? I'm not even sure that timezone is enough!

^{BTW, I'm not sure that Boost or C++11 <chrono> can reliably deal with such calendar issues.}

nwp mentioned in a comment a very good computerphile video: Problem with Time & Timezones.