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This C++ library converts precisely between timestamps in the timescale of UTC with smeared leap seconds and the unsmeared TAI and GPS timescales.

This allows smeared time to be stored and distributed exclusively, then converted to and from other timescales in applications where the smear's 11.6 ppm frequency change is consequential. No parallel time distribution systems are required, nor is any mechanism necessary for a system to record what timescale its system clock is using.

In other words, the smear is not a way of approximately smudging the clock; it is a defined, precise, and reversible conversion. Rather than having seconds of constant length and minutes with a variable number of seconds, there are always 60 seconds in a minute and the length of a second sometimes differs from those of TAI.

Unsmear is used for timekeeping on every Google production machine. Although it is stable and mature, we may change the details of the API based on feedback. Although this is not an officially supported Google product, you can reach us on the unsmear-discuss mailing list.

API overview

Unsmear's API works with and closely parallels the Abseil time library. It uses absl::Time for timestamps in the smeared timescale, and absl::Duration for intervals of possibly-smeared seconds. unsmear::TaiTime and unsmear::GpsTime represent times in TAI and GPST. unsmear::Duration measures intervals of constant-length SI seconds in Terrestrial Time. These new types offer the benefits of the Abseil time types while also allowing for correct and type-safe conversions between smeared and unsmeared timescales. The resolution of these types is 1 ns or better.

An unsmear::LeapTable tracks the leap seconds that have been added to UTC, allowing for smearing and unsmearing conversions between the three supported timescales. Leap tables can be serialized and deserialized using the protocol buffer in unsmear/leap_table.proto. A current table of leap seconds is provided in leap_table.textpb.

We strongly recommend distributing the leap table to your applications in binary protobuf format. Protobuf text format is intended primarily for checked-in configurations and human-readable debugging output.

During the period for which leap seconds are already announced, the methods Smear(), Unsmear(), and UnsmearToGps() will convert precisely between smeared UTC and the unsmeared timescales. This period extends some amount past the last leap second, since IERS Bulletin C announces when leap seconds will not happen as well as when they will.

The methods FutureProofSmear(), FutureProofUnsmear(), and FutureProofUnsmearToGps() will convert times after the expiration of the leap table by returning an interval of possible times.

Unsmear is built using the Bazel build system.


To perform conversions, construct a unsmear::LeapTable from the binary protobuf data:

std::ifstream stream;"leap_table.pb", std::ios::binary);
unsmear::LeapTableProto pb;
if (!pb.ParseFromIstream(&stream)) { /* error... */ }
std::unique_ptr<unsmear::LeapTable> lt = unsmear::NewLeapTableFromProto(pb);

The Unsmear() method returns an absl::optional, which will be absl::nullopt if outside the valid range of the leap table. Leap seconds have already been determined for 2017, so this will succeed:

absl::Time utc =
    absl::FromDateTime(2017, 1, 15, 10, 0, 0, absl::UTCTimeZone());
unsmear::TaiTime tai = *lt->Unsmear(utc);  // Note '*' to de-optionalize.

We can reverse the conversion using Smear():

absl::Time utc2 = *lt->Smear(tai);
assert(utc == utc2);

Conversions beyond the end of the leap table will return a range of times based on how many possible leap seconds may have occurred.

absl::Time utc = lt->expiration();
std::pair<unsmear::TaiTime, unsmear::TaiTime> tai_beyond =
    lt->FutureProofUnsmear(utc + absl::Hours(48));

Leap seconds may be inserted at the end of any UTC month, and may be positive or negative (i.e., seconds added or inserted). This results in the converted range expanding by one second in each direction for the 24-hour period crossing every possible smear. In this example, for a time 48 hours past the end of the leap table, there may have been one negative leap second, no leap second, or one positive leap second.

assert(tai_beyond.second - tai_beyond.first == unsmear::Seconds(2));
unsmear::TaiTime tai = *lt->Unsmear(utc);
assert(tai_beyond.first - tai == unsmear::Hours(48) - unsmear::Seconds(1));
assert(tai_beyond.second - tai == unsmear::Hours(48) + unsmear::Seconds(1));

This allows storage of future timestamps in the very popular representation as a count of seconds or nanoseconds since an epoch. As the time approaches, the leap seconds will be determined, and the window of conversion uncertainty will reduce to zero.

Additional API details

Valid ranges of times

The supported timescales are defined from these starting times:

1958-01-01 00:00:00 TAI n/a n/a
1972-01-01 00:00:10 TAI 1972-01-01 00:00:00 UTC n/a
1980-01-06 00:00:19 TAI 1980-01-06 00:00:00 UTC 1980-01-06 00:00:00 GPST

Like absl::Time, unsmear::TaiTime and unsmear::GpsTime times may be created with any value, including times before the beginning of the timescale. This allows ordinary laws of arithmetic to hold; for example:

unsmear::Duration d = unsmear::Seconds(1);
unsmear::TaiTime epoch = unsmear::TaiEpoch();
// Ok even though (epoch - d) is not a well-defined time:
unsmear::TaiTime t1 = epoch - d + d;

Active conversions using a LeapTable do not consider the timescales to be proleptic, and will only convert times after 1972-01-01 00:00:00 UTC, the start of modern UTC with leap seconds. When converting smeared UTC to or from GPST, the time must also not be before that epoch, 1980-01-06 00:00:00 GPST.

In particular, the Unix epoch of 1970-01-01 00:00:00 UTC predates modern UTC, and so it cannot be smeared or unsmeared. During the period between the Unix epoch and the modern UTC epoch, UTC seconds were not equal to TAI seconds, and a discontinuity of 107.758 ms was introduced. Smearing during this period is infeasible.

Smear() and Unsmear() report errors by returning an empty absl::optional. The FutureProof methods return an interval from the infinite past to the infinite future.

A leap table is not necessary to convert from TAI to GPST or the reverse. Use the ToTaiTime() and ToGpsTime() functions. There are no restrictions on what times may be converted in this way.

Formatting and parsing

unsmear::FormatTime() and unsmear::FormatDuration() will convert times and durations to strings.

The default format is ISO 8601, e.g., 2006-01-02 15:04:05.999999999 TAI. You can also provide a custom format string for times, supporting the same specifiers used by absl::FormatTime() (roughly, those used by strftime(3)). For convenience, unsmear::FormatTime() will also accept an absl::Time to be formatted as UTC.

The time and duration types can also be output to a stream:

std::cout << "Duration since TAI epoch is " << now - TaiEpoch();

unsmear::ParseDuration() will convert a string to an unsmear::Duration.

unsmear::Duration d;
if (!unsmear::ParseDuration(s, &d)) {
  std::cerr << "Invalid duration string: " << s << "\n";


JdnToTime() will convert an integer Julian Day Number to an absl::Time. Julian days begin at noon UTC.

absl::Time t = unsmear::JdnToTime(2451545);  // 2000-01-01 12:00:00 UTC

The command-line leap_table_tool will convert the provided text-format LeapTableProto in leap_table.textpb to a binary protobuf, to JSON, or to a human-readable debugging output.

leap_table_tool --output=proto leap_table.textpb > leap_table.pb
leap_table_tool --output=json leap_table.textpb > leap_table.json
leap_table_tool --input=proto --output=debug leap_table.pb

Smear details

The smear implemented here is from noon to noon UTC. The details and rationale are described at

This smear is identical to the smear used by Google Public NTP, by all other Google services, and by Amazon Web Services. This consistency is what allows the smear to be used interoperably and to be reliably reversed.

The concept of a "future-proof" API that provides a range of possible times based on unknown future leap seconds was described in the paper "Leap-smeared representation of time for high-accuracy applications", published by Google in 2016 as a defensive publication.


Converts to and from timescales with smeared leap seconds



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