25 Time library [time]

25.7 Clocks [time.clock]

25.7.4 Class gps_­clock [time.clock.gps]

25.7.4.1 Overview [time.clock.gps.overview]

namespace std::chrono {
  class gps_clock {
  public:
    using rep                       = a signed arithmetic type;
    using period                    = ratio<unspecified, unspecified>;
    using duration                  = chrono::duration<rep, period>;
    using time_point                = chrono::time_point<gps_clock>;
    static constexpr bool is_steady = unspecified;

    static time_point now();

    template<class Duration>
      static utc_time<common_type_t<Duration, seconds>>
        to_utc(const gps_time<Duration>&) noexcept;
    template<class Duration>
      static gps_time<common_type_t<Duration, seconds>>
        from_utc(const utc_time<Duration>&) noexcept;
  };
}
The clock gps_­clock measures seconds since the first Sunday of January, 1980 00:00:00 UTC.
Leap seconds are not inserted into GPS.
Therefore every time a leap second is inserted into UTC, UTC falls another second behind GPS.
Aside from the offset from 1958y/January/1 to 1980y/January/Sunday[1], GPS is behind TAI by 19s due to the 10s offset between 1958 and 1970 and the additional 9 leap seconds inserted between 1970 and 1980.
gps_­clock is not a Cpp17TrivialClock unless the implementation can guarantee that gps_­clock::now() does not propagate an exception.
[Note
:
noexcept(from_­utc(utc_­clock::now())) is false.
end note
]

25.7.4.2 Member functions [time.clock.gps.members]

static time_point now();
Returns: from_­utc(utc_­clock::now()), or a more accurate value of gps_­time.
template<class Duration> static utc_time<common_type_t<Duration, seconds>> to_utc(const gps_time<Duration>& t) noexcept;
Returns:
gps_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} + 315964809s
[Note
:
315964809s == sys_days{1980y/January/Sunday[1]} - sys_days{1970y/January/1} + 9s
end note
]
template<class Duration> static gps_time<common_type_t<Duration, seconds>> from_utc(const utc_time<Duration>& t) noexcept;
Returns:
gps_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} - 315964809s
[Note
:
315964809s == sys_days{1980y/January/Sunday[1]} - sys_days{1970y/January/1} + 9s
end note
]

25.7.4.3 Non-member functions [time.clock.gps.nonmembers]

template<class charT, class traits, class Duration> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const gps_time<Duration>& t);
Effects: Calls to_­stream(os, fmt, t), where fmt is a string containing "%F %T" widened to charT.
Returns: os.
template<class charT, class traits, class Duration> basic_ostream<charT, traits>& to_stream(basic_ostream<charT, traits>& os, const charT* fmt, const gps_time<Duration>& tp);
Effects: Streams tp into os using the format specified by the NTCTS fmt.
fmt encoding follows the rules specified in [time.format].
If %Z is used, it will be replaced with "GPS".
If %z is used (or a modified variant of %z), an offset of 0min will be formatted.
The date and time formatted shall be equivalent to that formatted by a sys_­time initialized with:
sys_time<Duration>{tp.time_since_epoch()} +
  (sys_days{1980y/January/Sunday[1]} - sys_days{1970y/January/1})
Returns: os.
[Example
:
auto st = sys_days{2000y/January/1};
auto gt = clock_cast<gps_clock>(st);
cout << format("%F %T %Z == ", st) << format("%F %T %Z\n", gt);
Produces this output:
2000-01-01 00:00:00 UTC == 2000-01-01 00:00:13 GPS
end example
]
template<class charT, class traits, class Duration, class Alloc = allocator<charT>> basic_istream<charT, traits>& from_stream(basic_istream<charT, traits>& is, const charT* fmt, gps_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);
Effects: Attempts to parse the input stream is into the gps_­time tp using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid date, is.setstate(ios_­base::failbit) shall be called and tp shall not be modified.
If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.
If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.
Additionally, the parsed offset will be subtracted from the successfully parsed timestamp prior to assigning that difference to tp.
Returns: is.