In a simple test getting the elapsed time in seconds since de Unix Epoch (01/01/1970 00:00:00 UTC) to 22 of February of 2019, it has returned a huge number of 18446744072104596016
Why not just 1550793600 seconds?
Isn't the time_t value portable?
The tm struct printed out in gdb console had the following values just before calling mktime():
(gdb) p ltm
$4 = {tm_sec = 0, tm_min = 0, tm_hour = 0, tm_mday = 22, tm_mon = 1, tm_year = 19, tm_wday = 0, tm_yday = 0, tm_isdst = 0}
Given
tm_year = 19
you are incorrectly using mktime() if you are expecting that input to represent the year 2019.
Per 7.27.1 Components of time, paragraph 4 of the C standard:
The tm structure shall contain at least the following members, in any order. The semantics of the members and their normal ranges are expressed in the comments.
int tm_sec; // seconds after the minute -- [0, 60] int tm_min; // minutes after the hour -- [0, 59] int tm_hour; // hours since midnight -- [0, 23] int tm_mday; // day of the month -- [1, 31] int tm_mon; // months since January -- [0, 11] int tm_year; // years since 1900 int tm_wday; // days since Sunday -- [0, 6] int tm_yday; // days since January 1 -- [0, 365] int tm_isdst; // Daylight Saving Time flag
Note that the tm_year value is denoted in years since 1900.
You are likely getting the time_t value for Feb 22, 1919, which is -1604966400, or 18446744072104596016 unsigned.