Short answer: A leap second changes how systems label and convert time, not the continuity of GPS’s underlying signal. GPS time runs continuously and keeps a known offset from UTC. Mobile networks can use that continuous reference for radio timing. Internet computers using NTP must choose a consistent policy—step, repeat, or smear the adjustment. Problems arise when equipment cannot represent 23:59:60, mixes incompatible policies, or applies an outdated GPS-to-UTC offset.
What a leap second actually changes
Coordinated Universal Time (UTC) is built from atomic clocks, but civil time is also required to track Earth’s irregular rotation. The International Earth Rotation and Reference Systems Service publishes UT1, an astronomical timescale derived from Earth rotation. UTC is adjusted so that the absolute difference between UT1 and UTC remains below 0.9 seconds. NIST defines a leap second as “a second added to Coordinated Universal Time (UTC) in order to keep it synchronized with astronomical time.”
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On the NIST page current in 2026, the listed UTC–TAI difference is 37 seconds, and the most recent leap second occurred on 2016-12-31. That 37-second figure is the accumulated difference between International Atomic Time (TAI) and UTC after all historical leap-second adjustments; it is not a countdown to the next event.
The unusual timestamp
A positive leap second is inserted after 23:59:59 UTC and before midnight. The sequence is:
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- 23:59:59
- 23:59:60
- 00:00:00 on the next UTC date
The label 23:59:60 is valid in UTC, but many software formats, databases, kernels and network protocols assume that every minute contains exactly 60 representable seconds. NIST notes that NTP cannot directly represent that label. An implementation may repeat 23:59:59, repeat the following 00:00:00, or distribute the extra second over a longer interval. Those choices preserve different properties, so two correctly configured systems can show different wall-clock readings temporarily.
Does a leap second affect GPS?
It affects GPS-to-UTC conversion, not GPS ranging itself. GPS time is a continuous navigation timescale. Unlike UTC, it does not insert leap seconds. ITU’s overview reports GPS time as 18 seconds ahead of UTC and explains that most global navigation satellite systems, excluding GLONASS, synchronized their clocks with UTC at the outset without adding leap seconds.
Why a receiver still needs leap-second data
A GPS receiver measures signal travel time and computes position using the continuous GPS timescale. That operation does not pause or repeat when UTC gains a leap second. If the receiver displays civil time, timestamps a measurement in UTC, or passes time to another system, it must apply the current GPS–UTC offset.
The offset changes only when UTC receives a leap second. A receiver therefore needs current navigation data, a maintained leap-second table, or firmware that can install new parameters. An old receiver may continue to calculate a perfectly usable position while reporting a UTC time that is wrong by one second after an adjustment.
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What users normally notice
- Ordinary navigation does not suddenly lose accuracy because UTC gained a second.
- UTC-stamped sensor records, financial transactions or scientific observations can be displaced if the offset table is stale.
- Equipment that disciplines another clock from GPS must document whether it outputs GPS time, UTC, TAI or a vendor-specific timescale.
In other words, GPS provides a stable reference; the conversion at the receiver or downstream application is where leap-second handling belongs.
Will a leap second break a mobile network?
Modern mobile systems are designed to keep radio timing continuous. Scheduling radio frames, aligning frequency and phase across cells, coordinating time-sensitive services and supporting positioning all depend on clocks that do not unexpectedly repeat a second.
ITU guidance quoting the 3GPP requirement says implementations need a continuous timescale. It gives GPS time as an example of UTC information that is not itself impacted by leap seconds when the reference is carried by a GPS signal. A network can therefore distribute a continuous reference internally and avoid putting the 23:59:60 discontinuity into the timing loop that controls the radio.
Where UTC still matters
Continuous radio timing does not eliminate UTC. A carrier still has to timestamp alarms, charging records, lawful-intercept data, maintenance logs and interfaces with external services. The safe design is to keep the continuous timescale in the timing core and perform UTC labeling at controlled boundaries.
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Typical failure modes
- Bad boundary conversion: a gateway applies the new offset twice or not at all, shifting records by one second.
- Mixed references: one subsystem follows stepped UTC while another follows a continuous GPS-derived source.
- Unsupported timestamp: a database or parser rejects 23:59:60, causing dropped alarms or failed transactions.
- Clock servo disturbance: a timing loop interprets a deliberate step as a large frequency error and over-corrects.
These are configuration and interoperability failures, not a loss of the satellite signal or a mandatory outage of cellular service.
How NTP handles a leap second
NTPv4, described in RFC 5905, is widely used to synchronize Internet computer clocks. NTP distributes UTC-related time, but the protocol and the operating system still need a policy for the extra second and for the fact that many timestamp formats cannot name it unambiguously.
Step or repeat
With a step-based policy, the clock is adjusted at the leap boundary. Depending on the operating system and daemon, the extra second may be represented by repeating 23:59:59, by a flag that tells the kernel a leap is in progress, or by a short discontinuity in the displayed clock. This approach keeps the clock close to legal UTC, but code that assumes wall time always increases can see duplicate timestamps. Event ordering and interval calculations are especially risky if an application subtracts two wall-clock values across the boundary.
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A smear changes the clock rate gradually so no single timestamp repeats. Google’s documented standard uses a linear 24-hour smear from noon to noon UTC and states that the method is used across its services. During the smear, the service clock is intentionally a small amount ahead of or behind legal UTC, reaching the full one-second difference at the midpoint before returning to alignment.
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Smearing is often attractive for distributed infrastructure because monotonically increasing wall time is easier for schedulers, databases and logs. NIST cautions, however, that a smear introduces a temporary time and frequency error relative to legal UTC. A smeared clock is therefore not interchangeable with an unsmeared UTC source during the smear window.
Hybrid and vendor-specific policies
Some kernels, cloud platforms and time services combine a leap indicator, a short rate adjustment and a later correction. The exact behavior depends on the implementation. What matters operationally is that every source feeding a cluster follows the same documented policy, or that an explicit conversion layer translates between policies.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why mixing time sources causes incidents
Suppose one server receives a noon-to-noon smear while another receives a stepped UTC feed. Their readings can differ by nearly a second during the smear. A load balancer may send an event to either host, producing log entries that appear out of order. A distributed lock can expire early on one node and late on another. A certificate, message signature or replay window based on wall time can be rejected even though each machine is behaving consistently with its own source.
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Step, smear or continuous reference: choosing an engineering policy
| Policy | Local-clock continuity | Fidelity to legal UTC | Can it represent 23:59:60? | Main interoperability risk | Good fit |
|---|---|---|---|---|---|
| Step or repeat | May repeat or jump at the boundary | Closest to UTC at the event | Often no; behavior is implementation-specific | Duplicate timestamps and applications that assume monotonic wall time | UTC-audited systems with leap-aware software |
| 24-hour smear | Continuous and usually monotonic | Temporarily differs from UTC by a controlled amount | No single repeated second | Disagreement with unsmeared NTP sources | Large distributed services whose partners use the same smear |
| Continuous GPS-derived reference | Continuous | Requires conversion before UTC is published | Not applicable to the internal timescale | Stale GPS–UTC parameters at the conversion boundary | Mobile-radio timing and other systems needing stable frequency and phase |
There is no universally correct choice. The requirement is consistency inside each failure domain and an explicit rule at every interface that crosses from one timescale to another.
How to prepare systems for a future leap second
- Inventory every clock. Record whether each device produces UTC, GPS time, TAI, a monotonic counter or a smeared civil clock. Include receivers, hypervisors, kernels, NTP clients, databases and telecom timing appliances.
- Choose one policy per cluster or fleet. Document whether the source steps, repeats or smears, and identify the exact smear interval if applicable. Do not leave the decision to a mixture of default operating-system settings.
- Separate elapsed time from wall time. Use a monotonic clock for timeouts, retry delays, lease expiration, media duration and performance measurements. Use UTC or another named civil timescale only when you need a human or external timestamp.
- Update GPS parameters. Verify receiver firmware and install current GPS-to-UTC leap information before an announced event. Confirm that downstream equipment does not overwrite the updated offset with an older table.
- Test the boundary and the smear window. Generate timestamps around 23:59:60 for step-based systems and across the full smear interval for smeared systems. Check ordering, uniqueness constraints, schedulers, retention jobs, billing, authentication windows and alert deduplication.
- Test loss and recovery. Disconnect the preferred time source, restart clients, and observe whether they fail safely or jump when they reacquire time. Verify that a timing loop does not treat the planned adjustment as a hardware fault.
- Label high-precision records. Store the time scale and source alongside the timestamp. A later analyst should be able to distinguish UTC, GPS time, TAI and a smeared civil clock without guessing.
- Audit external partners. Ask upstream NTP providers, cloud platforms, carriers and data feeds whether they smear or step. Add monitoring that raises an alert when two authoritative sources diverge beyond the permitted tolerance.
What history shows about leap-second risk
Leap seconds are infrequent, but rarity makes them easy to neglect. 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 demonstrates why a system can appear healthy for years and still fail when a rarely exercised code path is activated.
The lesson is not that every leap second causes a global outage. It is that timestamp parsing, clock discipline and source selection must be tested deliberately. A one-second disagreement can be material when it controls a timeout, orders transactions, aligns radio frames or determines whether a security token is valid.
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“Will my phone lose cellular service?”
Not merely because UTC gains a leap second. Cellular timing is engineered around a continuous reference, often one derived from satellite timing. Service can still be affected if a carrier has an implementation defect in its timing distribution or in UTC conversion at a network boundary.
“Will my GPS navigation suddenly be wrong?”
Position calculations should continue because GPS time itself is continuous. The displayed UTC time or a recorded timestamp can be wrong by one second if the receiver has not received or installed the new offset.
“Can I compare logs from two providers?”
Only after checking their time policies. Logs from a smeared source and an unsmeared source can disagree during the smear window even when both are synchronized correctly according to their respective rules.
“Should an application ever subtract two wall-clock timestamps?”
Not for elapsed-time logic across a possible leap event. Use a monotonic clock for durations and deadlines, and retain the wall-clock timestamp separately for display, audit or interoperability.
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