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Usually, a leap second does not interrupt the GPS signal or make a mobile network lose synchronization. It changes the relationship between civil UTC and continuous time scales. GPS time continues without inserting the extra second, mobile standards can distribute a continuous reference such as GPS time, and Internet systems using NTP must apply one consistent policy—step, repeat, or smear the adjustment.
The main risks are at conversion and software boundaries: a timestamp labeled 23:59:60 is difficult for many digital formats to represent, elapsed-time code can fail if it uses a wall clock, and a fleet that mixes smeared and unsmeared time sources can temporarily disagree by nearly a second. The last listed UTC leap second occurred on 2016-12-31; NIST’s 2026 page state lists UTC–TAI as 37 seconds and the intended UT1–UTC limit as ±0.9 seconds.
What a leap second is
Earth does not rotate at a perfectly constant rate. UTC is built from atomic time, but it is occasionally adjusted so that it stays close to UT1, the astronomical time scale that follows Earth’s actual rotation. The target is to keep the absolute difference between UT1 and UTC below 0.9 seconds.
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23:59:5923:59:60— the inserted second00:00:00— the next day
The label 23:59:60 is valid UTC notation, but many computer time formats and protocols were designed around 60-second minutes. NIST notes that NTP cannot directly represent that label. Implementations therefore have to choose how to encode the event: repeat 23:59:59, repeat the following day’s 00:00:00, or spread the correction over a longer period.
NIST’s current reference page lists a UTC–TAI difference of 37 seconds. That number is not the GPS offset; TAI, UTC, and GPS time are different scales. The 37-second value is the accumulated UTC adjustment relative to atomic International Atomic Time as listed by NIST, while the GPS-to-UTC relationship is maintained separately.
Do leap seconds affect GPS?
GPS time itself is not interrupted by a UTC leap second. It is a continuous navigation time scale. The International Telecommunication Union (ITU) reports that GPS time is 18 seconds ahead of UTC in its overview and explains that most global navigation satellite systems, excluding GLONASS, were synchronized with UTC at the outset without adding leap seconds.
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A GPS receiver can continue measuring signal timing and calculating ranges while UTC inserts a leap second. The receiver’s job is to convert the continuous navigation time into UTC only when an application needs civil time. That conversion requires the current GPS–UTC offset and an up-to-date leap-second parameter supplied through navigation data or a maintained table.
- Position and ranging: continuous GPS timing remains available; a leap label does not create a missing GPS signal.
- UTC display: the receiver must apply the current offset to show the correct civil date and time.
- Logging and synchronization: software must know whether a timestamp is GPS time, UTC, TAI, or a locally adjusted clock.
- Firmware maintenance: receivers need a way to accept new leap-second parameters before an announced event.
The offset changes only when UTC receives a leap second. A receiver with stale parameters can therefore keep producing plausible positions while reporting UTC incorrectly. That is a conversion error, not a failure of the underlying satellite ranging service.
Why GPS time is deliberately different from UTC
Navigation, communications, and control systems benefit from a time scale that never repeats or skips a second. GPS can provide that continuity. UTC remains essential for civil timestamps and for systems that must agree with legal or astronomical time, so equipment commonly keeps a continuous internal reference and performs UTC conversion at a defined interface.
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Will a leap second break a mobile network?
Normally, no—but mobile equipment must separate continuous radio timing from civil-time labeling. Radio scheduling, frequency and phase alignment, and other time-sensitive functions need a stable timescale. ITU mobile-network guidance quotes the 3GPP requirement for a continuous timescale and says an implementation can use UTC information that is not impacted by leap seconds, “e.g., GPS time in case the reference is carried by a GPS signal.”
How operators contain the change
A practical architecture distributes a continuous reference through the radio and transport network. GPS-derived time is one possible source; other timing designs can provide the same continuity. UTC is then handled at controlled boundaries such as management systems, event logs, billing records, public-facing clocks, and interfaces to external timing services.
- Radio layer: preserve a continuous reference for frame timing, scheduling, and synchronization.
- Core and transport: keep clocks aligned to the operator’s documented time policy.
- Operations and records: convert to UTC deliberately, with an unambiguous representation for the leap event.
- External interconnection: verify that neighboring networks and timing providers use compatible step or smear behavior.
UTC is not irrelevant to a carrier. A call record, alarm, lawful-intercept record, or inter-operator message may need civil-time traceability. The engineering objective is to avoid allowing the UTC label change to disturb functions that require continuity.
Where failures can still occur
Problems arise when one subsystem assumes a repeated second and another assumes a smooth clock, or when a device loses the leap parameters used for UTC conversion. Schedulers, database indexes, charging systems, and monitoring tools can show duplicate timestamps or appear to process events out of order even though the radio timing reference remains stable.
How Internet clocks and NTP handle a leap second
Network Time Protocol version 4 (NTPv4), described in RFC 5905, is widely used to synchronize Internet computers. NTP distributes time, but it does not force every operating system, kernel, cloud platform, or service to represent a leap second in the same way. Each implementation needs a documented policy.
Step or repeat the second
In a step-based approach, the system applies the adjustment at the UTC boundary. Because many timestamp formats cannot encode 23:59:60, software may see a repeated 23:59:59, an ambiguous boundary, or a clock correction immediately before or after the event.
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This method keeps the clock close to legal UTC at the boundary, but it can make wall-clock time non-monotonic. Code that assumes every later timestamp is greater than every earlier timestamp can misorder events, calculate a negative duration, or trigger a timeout twice.
Smear the adjustment
A smear changes the clock rate gradually, avoiding a visibly repeated second. Google’s documented standard uses a linear 24-hour smear from noon to noon UTC and says the approach has been used across its services since 2008 instead of applying leap seconds with clock steps.
During the smear window, the local clock is intentionally offset in both time and frequency from legal UTC. NIST warns that this creates a temporary time and frequency error relative to UTC. The benefit is a continuously increasing local clock that is easier for distributed applications to consume.
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Hybrid or vendor-specific behavior
Operating systems, kernels, cloud platforms, and time services may implement variants of stepping, repeating, or smearing. A hybrid policy can be safe when its boundaries are explicit—for example, a smear inside a controlled fleet with a conversion layer at the edge. It is unsafe when different components silently select different policies.
Why mixing smeared and unsmeared sources causes trouble
Two NTP sources can both be healthy and still disagree during a smear. The smeared source is intentionally moving gradually, while the unsmeared source remains aligned to UTC and handles the leap at the boundary. A client that switches between them can see a sudden offset, oscillating corrections, or an apparent time jump.
Google specifically warns that smeared and unsmeared sources can disagree during the smear. ITU guidance also reports that stepping can cause failures in data-center systems and that smearing is often preferred for distributed infrastructure. The choice is less important than consistency: every clock in a cluster, or a clearly defined conversion layer, must understand the same scale.
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Compare the main implementation choices
| Policy | Local-clock continuity | Fidelity to legal UTC | Timestamp representation | Interoperability risk | Typical workload fit |
|---|---|---|---|---|---|
| Step or repeat | May pause, repeat, or jump at the event | Closest to UTC at the boundary | Must handle an ambiguous or unrepresentable leap label | High if peers use a smear | Systems with explicit leap-second support and strict UTC traceability |
| Smear | Continuous and monotonic when implemented correctly | Temporarily differs from legal UTC in time and rate | No single repeated second is exposed | High if peers use unsmeared UTC | Large distributed fleets that standardize one smear window |
| Hybrid with a conversion boundary | Continuous internally, deliberate conversion externally | Depends on the documented conversion point | Can preserve both internal continuity and UTC records | Manageable only with explicit metadata and testing | Telecom and enterprise systems spanning continuous and civil-time domains |
For mobile-radio functions, continuity is usually the governing requirement. For legal records, scientific correlation, and cross-organization exchange, UTC fidelity and clear scale labeling matter more. A design can support both, but it should not pretend that one unlabeled timestamp satisfies every purpose.
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What happens to applications at the event
Elapsed-time and timeout logic
Use a monotonic clock for durations, retry intervals, leases, and timeout decisions. Do not subtract two wall-clock timestamps across a leap event and assume the result is an elapsed interval. Wall clocks can repeat or be gradually adjusted; a monotonic source is intended to move forward for exactly this kind of logic.
Logs and event ordering
Store the time scale and source with high-precision records. A log entry should make it possible to distinguish UTC, GPS time, TAI, and a smeared civil clock. If two events share the same displayed second, use a sequence number, monotonic counter, or higher-resolution ordering field rather than relying on the civil label alone.
Databases and schedulers
Test unique timestamp keys, retention jobs, cron-like schedules, cache expiry, certificate checks, and message ordering around the leap boundary or the selected smear window. A repeated civil timestamp can create duplicate-key errors; a smeared clock can delay or advance a scheduled action relative to an unsmeared external system.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Engineering checklist for a leap-second-ready system
- Choose one policy. Document whether each environment steps, repeats, smears, or converts at a boundary.
- Inventory every time source. Identify NTP servers, GPS receivers, operating-system clocks, cloud time services, and vendor appliances.
- Prevent mixed inputs. Do not combine smeared and unsmeared NTP sources unless an explicit conversion layer accounts for the difference.
- Maintain GPS parameters. Update GPS-to-UTC leap information and verify receiver firmware before an announced event.
- Separate clocks by purpose. Use monotonic time for elapsed-time logic and a labeled civil clock for human-readable records.
- Test the boundary. Exercise timestamp ordering, logs, databases, schedulers, telecom control functions, and failover behavior around
23:59:60or the chosen smear interval. - Record metadata. Include the time scale, source, precision, and policy in high-precision telemetry so later analysis can reconstruct what the system believed.
- Plan degraded operation. Decide what happens if a receiver has stale leap data, an NTP source disappears, or a node changes policy during maintenance.
What the 2012 event teaches
Leap-second handling is not merely theoretical. GPS.gov’s Civil GPS Service Interface Committee reported that approximately 10% of the world’s NTP servers failed to handle the 2012 insertion correctly. The incident illustrates why a service can appear healthy during ordinary operation yet fail when a rare calendar boundary exercises an untested code path.
GPS-dependent systems can also be affected indirectly: the continuous signal may remain available while software that converts it to UTC uses outdated parameters or assumes that every minute has the same representable labels. The right response is versioned configuration, controlled testing, and clear identification of the time scale—not disabling accurate timing altogether.
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Direct answers to the common questions
Do leap seconds affect GPS?
They affect the GPS-to-UTC conversion, not the continuity of GPS time or the basic ranging signal. Receivers and downstream software need current leap-second parameters when they produce UTC.
Will a leap second break my phone network?
It should not if the network uses a continuous reference for radio and timing functions and handles UTC conversion at controlled interfaces. Failures are more likely in logging, scheduling, charging, or inter-system conversion than in the radio signal itself.
How does NTP handle a leap second?
NTP implementations step or repeat the boundary, smear the correction over a defined interval, or use a documented vendor-specific variant. The entire fleet must use compatible sources.
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Why is GPS time different from UTC?
GPS time is designed as a continuous navigation scale and does not insert UTC leap seconds. UTC is adjusted to remain close to Earth-rotation time, so the two scales maintain a changing, explicitly managed offset.
What happens to Internet clocks when a leap second is added?
Some clocks repeat or step at the UTC boundary; others gradually adjust over a smear window. Applications that assume wall-clock monotonicity or mix the two policies can misorder events or calculate incorrect intervals.
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