Will `gmtime()` report seconds as 60 when in a leap second?

The short answer is, no, practically speaking gmtime_r will never fill in tm_sec with 60. This is unfortunate, but unavoidable.

The fundamental problem is that time_t is, per the Posix standard, a count of seconds since 1970-01-01 UTC assuming no leap seconds.

During the most recent leap second, the progression was like this:

1483228799    2016-12-31 23:59:591483228800    2017-01-01 00:00:00

Yes, there should have been a leap second, 23:59:60, in there. But there's no possible time_t value in between 1483228799 and 1483228800.

I know of two ways for a gmtime variant to return a time ending in :60:

1. You can run your OS clock on something other than UTC, typically TAI or TAI-10, and use the so-called "right" timezones to convert to UTC (or local time) for display. See this web page for some discussion on this.

2. You can use clock_gettime() and define a new clkid value, perhaps CLOCK_UTC, which gets around the time_t problem by using deliberately nonnormalized struct timespec values when necessary. For example, the way to get a time value in between 1483228799 and 1483228800 is to set tv_sec to 1483228799 and tv_nsec to 1000000000. See this web page for more details.

Way #1 works pretty well, but nobody uses it because nobody wants to run their kernel clock on anything other than the UTC it's supposed to be. (You end up having problems with things like filesystem timestamps, and programs like tar that embed those timestamps.)

Way #2 is a beautiful idea, IMO, but to my knowledge it has never been implemented in a released OS. (As it happens, I have a working implementation for Linux, but I haven't released my work yet.) For way #2 to work, you need a new gmtime variant, perhaps gmtime_ts_r, which accepts a struct timespec instead of a time_t.

Addendum: I just reread your question title. You asked, "Will gmtime() report 60 for seconds when the server is on a Leap Second?" We could answer that by saying "yes, but", with the disclaimer that since most servers can't represent time during a leap second properly, they're never "on" a leap second.

Addendum 2: I forgot to mention that scheme #1 seems to work better for local times -- that is, when you're calling one of the localtime variants -- than for UTC times and gmtime. Clearly the conversions performed by localtime are affected by the setting of the TZ environment variable, but it's not so clear that TZ has any effect on gmtime. I've observed that some gmtime implementations are influenced by TZ and can therefore do leap seconds in accordance with the "right" zones, and some cannot. In particular, the gmtime in GNU glibc seems to pay attention to the leap second information in a "right" zone if TZ specifies one, whereas the gmtime in the IANA tzcode distribution does not.

The question is will tm.tm_sec == 60 when the server is within a leap second?

No. On a typical UNIX system, time_t counts the number of non-leap seconds since the epoch (1970-01-01 00:00:00 GMT). As such, converting a time_t to a struct tm will always yield a time structure with a tm_sec value between 0 and 59.

Ignoring leap seconds in time_t reckoning makes it possible to convert a time_t to a human-readable date/time without full knowledge of all leap seconds before that time. It also makes it possible to unambiguously convert time_t values in the future; including leap seconds would make that impossible, as the presence of a leap second isn't known beyond 6 months in the future.

There are a few ways that UNIX and UNIX-like systems tend to handle leap seconds. Most typically, either:

1. One time_t value is repeated for the leap second. (This is the result of a strict interpretation of standards, but will cause many applications to malfunction, as it appears that time has gone backwards.)

2. System time is run slightly slower for some time surrounding the leap second to "smear" the leap second across a wider period. (This solution has been adopted by many large cloud platforms, including Google and Amazon. It avoids any local clock inconsistencies, at the expense of leaving the affected systems up to half a second out of sync with UTC for the duration.)

3. The system time is set to TAI. Since this doesn't include leap seconds, no leap second handling is necessary. (This is rare, as it will leave the system several seconds out of sync with UTC systems, which make up most of the world. But it may be a viable option for systems which have little to no contact with the outside world, and hence have no way of learning of upcoming leap seconds.)

4. The system is completely unaware of leap seconds, but its NTP client will correct the clock after the leap second leaves the system's clock one second off from the correct time. (This is what Windows does.)

There is absolutely no easy answer to this. For there to be a 60 second when there is a leap second, you require 1) something in the OS to know there is a leap second due, and 2) for the C library that your using to also know about the leap second, and do something with it.

An awful lot of OSes and libraries don't.

The best I've found is modern versions of Linux kernel teamed up with gpsd and ntpd, using a GPS receiver as the time reference. GPS advertises leap seconds in its system datastream, and gpsd, ntpd and the Linux kernel can maintain CLOCK_TAI whilst the leap second is happening, and the system clock is correct too. I don't know if glibc does a sensible thing with the leap second.

On other UNIXes your mileage will vary. Considerably.

Windows is a ******* disaster area. For example the DateTime class in C# doesn't know about historical leap seconds. The system clock will jump 1 second next time a network time update is received.