difftime returns strange value on particular date

Question

I don't understand why difftime returns a strange value, so here is the dataset and the code I am using.

Code :

struct tm currentTime;
currentTime.tm_year = 2014 - 1900;
currentTime.tm_mon = 9 - 1;
currentTime.tm_mday = 6;
currentTime.tm_hour = 23;
currentTime.tm_min = 59;
currentTime.tm_sec = 0;
currentTime.tm_wday = 7 - 1;

struct tm previousTime;
previousTime.tm_year = 2014 - 1900;
previousTime.tm_mon = 9 - 1;
previousTime.tm_mday = 6;
previousTime.tm_hour = 23;
previousTime.tm_min = 58;
previousTime.tm_sec = 0;
previousTime.tm_wday = 7 - 1;

cout << difftime(mktime(&currentTime), mktime(&previousTime)) << endl;

This prints :

Any ideas why I get this value ? I should get 60, as there is a one minute difference. I tried with some other values, and they all work.. I'm using CodeBlocks with mingw.

EDIT : Answer : use of tm_isdst solved the problem ! Bloody DST :P

struct tm currentTime;
currentTime.tm_year = 2014 - 1900;
currentTime.tm_mon = 9 - 1;
currentTime.tm_mday = 6;
currentTime.tm_hour = 23;
currentTime.tm_min = 59;
currentTime.tm_sec = 0;
currentTime.tm_wday = 7 - 1;
currentTime.tm_isdst = - 1;

struct tm previousTime;
previousTime.tm_year = 2014 - 1900;
previousTime.tm_mon = 9 - 1;
previousTime.tm_mday = 6;
previousTime.tm_hour = 23;
previousTime.tm_min = 58;
previousTime.tm_sec = 0;
previousTime.tm_wday = 7 - 1;
previousTime.tm_isdst = - 1;

cout << difftime(mktime(&currentTime), mktime(&previousTime)) << endl;

Aside from the fact that this doesn't compile [because the member names are incorrect](http://en.cppreference.com/w/cpp/chrono/c/tm), once you fix the errors [you get 60 as expected.](http://coliru.stacked-crooked.com/a/af76e98edee8cdbb). tl;dr; cannot repro. — Borgleader, Nov 06 '14 at 20:09
@Borgleader Yes sorry I didn't copy well my program on SO. I updated it with the new values, and I still get a strange value : 3660 — Jonathan Taws, Nov 06 '14 at 20:12
So you actually have `previousTime.tm_hour = 22;` in your code. — Anton Savin, Nov 06 '14 at 20:16
@AntonSavin No, I did the test with the exact same code than Borgleader posted, and I got 3660 (I just changed the years with a minus 1900 instead of 1990). It might come from mingw ? — Jonathan Taws, Nov 06 '14 at 20:17
It's more likely to be timezone related. Did you notice that your output is precisely one day out from what you wanted it to be? — Lightness Races in Orbit, Nov 06 '14 at 20:19
@LightnessRacesinOrbit Actually it's an hour out from what I want, that's really really strange.. — Jonathan Taws, Nov 06 '14 at 20:21
@Hawknight: Haha oops yeah an hour not a day. So the problem is likely even more simple than I thought. — Lightness Races in Orbit, Nov 06 '14 at 20:24
@Hawknight initialize `tm_isdst` field of both variables and see what happens — Anton Savin, Nov 06 '14 at 20:25
`mktime()` require 7 fields to be set, y-m-d, h:m:s and `is_dst`. Set to -1 if unsure. `tm_wday` is irrelevant. — chux - Reinstate Monica, Nov 06 '14 at 20:27
I initialized to zero tm_isdt in both, and it returns the correct time ! Thanks guys ! I then had a look and previousTime had a strange value set, while currentTime didn't, that was the problem ! — Jonathan Taws, Nov 06 '14 at 20:29
@Hawknight if you know DST was not applicable, using 0 is fine, otherwise use -1. — chux - Reinstate Monica, Nov 06 '14 at 20:32

chux - Reinstate Monica · Accepted Answer · 2014-11-06T20:57:23.697

Before calling mktime(), typically 7 fields of struct tm need to be set. As OP only set 6 of those, uninitialized data was in field tm_isdst causing an unexpected shift of 3600 seconds.

struct tm currentTime;
currentTime.tm_year = 2014 - 1900;
currentTime.tm_mon = 9 - 1;
currentTime.tm_mday = 6;
currentTime.tm_hour = 23;
currentTime.tm_min = 59;
currentTime.tm_sec = 0;

currentTime.tm_isdst = -1;  // **
// currentTime.tm_wday = 7 - 1; 

mktime(&currentTime);

Recommend to zero-fill struct tm as in struct tm currentTime = { 0 }; to insure all fields are specified as struct tm may contain fields in addition to the 9: int tm_sec tm_min tm_hour tm_mday tm_mon tm_year tm_wday tm_yday tm_isdst.

Notes:

The original values in tm_wday and tm_yday are ignored and re-calculated by mktime(). The other fields' original values are not restricted to their normal range and are recalculated too.

** a positive or zero value for tm_isdst causes the mktime function to presume initially that Daylight Saving Time, respectively, is or is not in effect for the specified time. A negative value causes it to attempt to determine whether Daylight Saving Time is in effect for the specified time.

Great explanation here. Basically, it's safer to zero-fill the variable before using it ? — Jonathan Taws, Nov 06 '14 at 21:18
@Hawknight Since `struct tm` has at least the well-defined 9 fields, portable code should not risk leaving others uninitialized. In general, when using structures not-of-your-making, best to zero them before using. Had your code zero-filled, things would have been as expected, except near DST shifts. Without digging deeper, use `tm_isdst = 0 or 1` when you know the setting, else use `-1`. — chux - Reinstate Monica, Nov 06 '14 at 21:26

difftime returns strange value on particular date

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