c++ - Why does tm_sec range from 0-60 instead of 0-59 in time.h?

Question

My time.h has the following definition of tm:

struct tm {
    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] */
    ...
}

I just noticed that they document tm_sec as ranging between 0-60 inclusive. I've always assumed it ranged from 0-59 just like tm_min. I've certainly never seen a clock read 10:37:60...

Do you think this is just a documentation bug left over from this 90's era Berkley-originated file?

Or is there something more subtle going on that I'm unaware of?

Answer 1

Leap seconds are the reason for this:

A leap second is a plus or minus one-second adjustment to the Coordinated Universal Time (UTC) time scale that keeps it close to mean solar time.

When a positive leap second is added at 23:59:60 UTC, it delays the start of the following UTC day (at 00:00:00 UTC) by one second, effectively slowing the UTC clock.