Why does mktime() need to know about daylight saving time? - c

Question

So mktime() returns a time_t value which is defined as "an integral value representing the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC" (source). I can convert a date to a time_t value by using mktime(). For example, to convert the date 10-Sep-2017 08:34:56 to a time_t I'd do the following:
struct tm tm;
time_t tv;
tm.tm_sec = 56;
tm.tm_min = 34;
tm.tm_hour = 8;
tm.tm_mday = 10;
tm.tm_mon = 8;
tm.tm_year = 117;
tm.tm_isdst = ????;
tv = mktime(&tm);
Now what I don't understand is the idea behind the tm_isdst parameter: It is described as "a flag that indicates whether daylight saving time is in effect at the time described" (source).
This description is somewhat confusing me because I'm of the opinion that the time I'm describing in struct tm is in fact already a UTC time and the time_t value I want to have from mktime() is UTC as well. But UTC time doesn't change with a change of seasons, so why does mktime() need to bother with daylight saving time at all? Isn't the advantage of using UTC instead of local time that I won't have to bother with daylight saving time? So why do I have to set tm_isdst then?
I'm sure the answer is really simple but at the moment I'm failing to see it. Could somebody please provide a simple example that illustrates why mktime() needs the tm_isdst parameter to convert a certain date and time to a time_t value?

Answer 1

Whether DST is in effect changes what the epoch time will be because mktime uses the current time zone to determine the time.
For example, if I populate tm with 1/1/70 00:00:00 as follows:
tm.tm_sec = 0;
tm.tm_min = 0;
tm.tm_hour = 0;
tm.tm_mday = 1;
tm.tm_mon = 0;
tm.tm_year = 70;
tm.tm_isdst = 0;
I get a value of 18000 for tv because my timezone is GMT-5 (18000 = 3600 * 5). If I change the value of tm_isdst to 1, then tv will be set to 14400 (3600 * 4).
Setting tm_isdst to -1 will look at the local timezone database to see if DST is in effect for the given date/time.

Answer 2

Now what I don't understand is the idea behind the tm_isdst parameter:
It is described as "a flag that indicates whether daylight saving time
is in effect at the time described" (source).
This description is somewhat confusing me because I'm of the opinion
that the time I'm describing in struct tm is in fact already a UTC
time
Well, that's your main problem. Per its documentation, which you already linked, mktime() converts a time structure expressed as local time to a time_t.
and the time_t value I want to have from mktime() is UTC as well.
Again according to the docs, when a time_t is interpreted as an absolute time, it represents the number of seconds elapsed since the Epoch, in UTC, so that's indeed what you will get from mktime(). But that means that there is, in general, a time zone and possibly daylight-saving difference between the two representations.
The C library relies on contextual localization data for the time zone applicable to local time, but although it knows the current rules for daylight saving time, it does not necessarily know the rules that were in effect at the specified time. Moreover, there are edge cases at the changeovers between daylight time and standard time. For these and perhaps other reasons, you need to tell mktime() whether the time given refers to daylight saving time or standard time.
But UTC time doesn't change with a change of seasons, so why does
mktime() need to bother with daylight saving time at all?
Because mktime() accepts local time as input.
Isn't the
advantage of using UTC instead of local time that I won't have to
bother with daylight saving time?
That is one of the advantages, yes.
So why do I have to set tm_isdst
then?
Because mktime() accepts local time as input.

Answer 3

struct tm can be thought of as a timestamp.
Example: When a timezone goes from daylight time to standard time, the "day" has 25 hours and struct tm needs more info than just "Y-M-D h:m:s".
The local time may be 2:30 AM daylight time and then 1 hour later it is 2:30 AM standard time. .tm_isdst is especially useful to distinguish that case. Note that .tm_isdst is relevant in all timestamps.
As struct tm itself does not contain the timezone (that info exist elsewhere), .tm_isdst is designed to provide time stamps indication of standard time or daylight time (or even unknown standard/daylight).
If the .tm_isdst is set to -1 (unknown) and the timezone is UTC, then the expectation is that, for all timestamps, it is as if .tm_isdst == 0.