C++ function that converts time(NULL) to local time

Question

In C++ I'm writing a function that converts time(NULL), which is all the seconds since January 1, 1970 in UTC time, to local time EST in military time format (hours:minutes:seconds). I'm honestly stumped how to mathematically do this so that the program stays accurate as time moves forward.

Also I'm well aware that there is a local time function but I'd like to build this function from the ground up. Does anyone have any advice or tips?

Answer 1

Also I'm well aware that there is a local time function but I'd like to build this function from the ground up. Does anyone have any advice or tips?

Why would you want to do this when there are plenty of free and well-tested packages? As mentioned in the comments, getting daylight savings time correct is non-trivial. Existing packages do just that, and they do it right, based on the IANA tzinfo database.

C options:

• std::localtime(). This function uses a global variable; it is not thread safe.

• localtime_r(). This is a POSIX function and is not a part of the C++ library. It does not exist on Windows.

• localtime_s(). This is an optional C11 function. Even if it exists on your machine, it might not be a part of <ctime>.

C++ options:

• Boost Date-Time, https://github.com/boostorg/date_time .

• Howard Hinant's date-time module, https://github.com/HowardHinnant/date .